Small design tweaks to the group share page

- Put a <br> between the document link text and the document hostname
  (if there is one)
- Double the amount of vertical space between each <li> in the list of
  documents
- Increase the number of characters after which document link text or
  hostname is truncated

Fix display of num replies on collapsed replies

When you collapse a reply that itself has replies an "n replies" link
appears next to the username on the collapsed reply. A recent commit
broke the display/alignment of this. Correct it.

Fixes #2702.

Avoid losing unsaved changes to new or existing annotations when switching groups

 * Avoid a loop over an array with a single entry

 * Avoid an unnecessary variable shared between multiple
   tests

Update widget-controller following drafts service API change.

When signing out whilst a group is selected and
unsaved drafts were present, the following would
happpen:

 1. User prompted to discard drafts

 2. User accepts prompt and draft store is cleared

 3. Group focus changes and a new draft is created

Fix this by only updating a draft when the group
focus changes, not creating a new one if it has been removed

The logic for handling this case in the controller is slightly
indirect. The draft is discarded not when the call to update
the annotation on the server succeeds but whenever the client
is notified that an annotation update has been committed,
signified via its 'updated' property changing.

This happens both for saves triggered locally and also
updates made in other H instances.

Refactor the drafts service to preserve unsaved edits
when switching groups.

This rewrites the drafts service so that it can preserve
unsaved changes for new and existing annotations when
switching groups.

For each annotation, the drafts service now maintains
an object containing the unsaved changes in addition to
the model for the annotation which was being edited.

 * For new annotations, the annotation is moved to the current
   group when switching groups.

 * For edits to existing annotations, the unsaved changes are
   saved to the drafts service.

 * When an annotation card is created, switch to editing mode
   automatically if a draft is present.

 * Avoid automatically discarding the draft when an annotation
   is unloaded. This allows unsaved edits to existing annotations
   in a group to be restored when switching back to the group.

Avoid route reload on change

Fix search button not expanding search bar in Safari

The search bar expands from 0 to 150px when the input is focused
but in Safari the <input> will not accept focus via
input.focus() if its max-width is _exactly_ 0px.

Setting it to a near-zero +ve value instead avoids the problem.

Fixes #2654

Add request context to errors caught by Sentry

Merge pull request #2687 from hypothesis/github-2686-fix-invalid-permissions-when-creating-annotations-when-signed-out

Set permissions of new annotations on login

Move the responsibility for creating the default permissions for new
annotations according to the cached-in-local-storage shared/private setting
into permissions.coffee, where it's with other permissions related stuff and
can be tested more easily.

While signed out create a new annotation. You'll see a "You must sign in
to create an annotation" card. This card is in fact an annotation
in the client-side code, but one that has no user, no permissions, etc
etc. If you sign in, you'll then see a full annotation card with editor
open in place of the previous card, you can type an annotation and save
it.

Except that this will send invalid permissions to the server, which will
happy save them (no validation) and then crash.

The fix:

On sign in, set the model.permissions of such annotations. This means
that the Post dropdown will set itself correctly to shared or private
and on posting the annotation the correct permissions will be sent to
the server.

Also fixed permissions.coffee to return null instead of invalid
permissions when not signed in.

Fixes #2686.

Add documentation explaining what the threading service
does and links explaining the 'jwz' message threading
algorithm and a reference to the JS implementation that
we are using.

When the focused group changes, instead of reloading the whole
view just unload the currently displayed annotations and then
reload the annotations for the newly selected group(s) and
any drafts of unsaved annotations.

The only immediately 'visible' effect of the change is
that switching groups now only results in a call to /search
rather than network requests to fetch session state and features.

In future this should make it easier to preserve state
for the area below the top bar when switching groups.

Fix 'Post to' button not updating when an annotation's group changed & cleanup code

The 'group' property on the scope used by the 'Post to' button
was set when the component was instantiated but not updated
if the group subsequently changed.

Fix this and simplify the code by using the 'bindToController'
facility from Angular 1.3.x to avoid copying properties
from the directive's scope to the controller.

This means that 'vm.group' expressions in the controller now
update when the corresponding attribute changes.

Adjust annotation card style to fit groups design

Query tags field with a match query (use mapping analyzer)

 * Move the group name and scope indicator onto a line
   below the username

 * Make the group name a lighter gray in the default state

 * Adjust the margins around the group card

 * Use a consistent font size from the typography palette
   for all text in the annotation card header.

 * Remove the 'Only Me' text next to the lock badge.
   Along with everything else in the header this added
   too much noise.

 * Increase the spacing between the group name and
   the excerpt

 * Fix unnecessary margin above annotations with no quotes
   by hide the annotation quote list section if an annotation
   does not have quotes.

 * Add a minimum margin between the annotation heading and the editor

T-148

Wait for feature flag data before loading sidebar view

Document link refactor

Clear the features cache only when the logged-in user changes,
rather than on any change to session data.

Feature flags can affect the initial search query when
the app loads. eg. If the groups feature is enabled then
we will by default load annotations for the most recently
used group, otherwise we'll load public annotations.

Therefore, wait for the feature flags to be retrieved
before loading the view.

The feature flags endpoint is very fast so the behavior
was correct most of the time previously but occassionally
the app would load and fetch Public annotations before
the feature data was available which would have resulted
in fetching group annotations.

At the same time, this clears the feature flag data
when the current user changes and fixes an issue where
switching from a non-groups enabled account to a groups-enabled
account did not immediately update the UI.

Fixes #2675

Fix exception when GROUPS_CHANGED event is broadcast during route load

The <group-list> directive attempted to update itself in
response to a group change notification without triggering
a full digest cycle by using `$scope.$apply`.

This was based on the incorrect understanding that $apply only
dirty-checks the current scope downwards. In fact, in dirty-checks
the root scope. Additionally, the logic was pointless since
group list/focus changes happen in response to two types of events,
both of which are triggered in the context of $apply:

 * An event handler when the user selects a group

 * A callback from angular-websocket when a WebSocket message
   is received.

Avoid route reload on groups list change

Angular 1.4.x introduced a breaking change (undocumented in the
upgrading guide as far as I can tell) to the way URL query parameters
are handled in ajax requests.

Specifically, semicolons in query parameter values are no longer encoded
by default. This causes problems for us in request urls such as

    /api/search?uri=http:%2F%2Fexample.com/?id=4;display=print

because Pyramid interprets the semicolon (correctly according to
RFC3986) as a query string delimiter.

This commit fixes the issue by overriding the default parameter
serializer (although only for the ngResource objects in the store
service) with a much more conservative one that encodes everything with
`encodeURIComponent`.

The bulk of the code here is a slightly modified version of the default
serializer used by Angular.

Avoid maintaining a copy of the groups list and focused group
states in the directive as per the discussion at
https://hypothes-is.slack.com/archives/public/p1446048142004464

A style for anchor tags was moved from `common.scss` to `elements.scss`,
and so we now need to include that here.

Remove the remnants of a defunct annotation pulse feature