If a session load fails, subsequent calls to
session.load() would trigger another request immediately.
Together with feature flag checks triggering session.load(),
this could result in a cycle if no network connection was present:

 1. App loads during initial digest cycle, 'flag.featureEnabled()'
    is called to test whether to show certain UI elements
 2. session.load() is triggered
 3. session.load() fails and the HTTP response error triggers
    a digest cycle
 4. GOTO 1

This commit resolves the problem by automatically retrying
session.load() calls with an exponential backoff.

Standalone websocket server

Group page update

Add help text to nipsa form

Fix client-side router on site

Don't 404 if a user visits an activation link twice

Do not update the group for existing annotations on edit

Fix flash of 'Create Account' banner in stream

Admin account deletion

Remove the need to specify the Chrome extension ID when building with bundled assets

Whenever an annotation was edited, its current group
was set to whichever group was focused in the UI.

Our current model does not allow for moving annotations
between groups however. The only time when annotations
should have their groups set is when the annotation
is newly created and when switching groups before saving
the annotation. We already have separate logic to deal
with both of these cases.

Fixes #2902

Fix user annotation query in admin panel

The JS for the group share page failed to load
because of an incorrect comparison of the result
of String.match() against a number and also
because of an attempt to load Bootstrap
unnecessarily on this pages.

This commit fixes the issue and prepares us for
adding new pages in future by:

 * Adding a client-side micro-router to simplify running
   page-specific JS

 * Only loading the Bootstrap components on pages that
   actually need it.

Fixes #2867

Make connecting the WebSocket optional

In preparation for splitting the hosting of the WebSocket server into a
separate process, make it possible to run the client without attempting
to connect to the WebSocket.

If the `websocketUrl` setting is missing, we will not attempt to
connect.

The WebSocket only accepts connections from a whitelist of
origins. When connecting to a local H service, provide
a useful hint when connections (eg. from a local Chrome
extension build with bundled assets) are rejected due
to the H service not having the sidebar app's origin
whitelisted.

Add YAPF config and docs for automatic formatting of Python source code

Double the truncation threshold height for annotations

Prevent double-posting of new annotations

As per https://trello.com/c/7QtpfMB5/ increase the threshold
so that only very long annotation bodies are truncated.

When the user clicks the 'Post' button to create an annotation,
optimistically switch the card back to View mode but display
a 'Saving...' indicator in place of the Reply/Edit/Delete links.

This makes the UI appear more responsive when the user clicks
the Post button and also prevents an issue where the user could
click 'Post' multiple times during the save and create multiple
annotations.

 * Fix a possible inconsistency between the 'Post' button's enabled
   state and whether or not the save() function can succeed.

   The hasContent() and isShared() methods also already have tests,
   so this lets us remove several redundant tests.

 * Fix inconsistency in the return type of the save() function -
   always return a promise.

 * Treat negative status values as network errors as well as 0.
   If the server is unreachable, the real status value may be -1.

Fixes #2864

Decouple sidebar app HTML / embed JS template rendering from Pyramid

Don't open "welcome page" when installed by admin policy

Decouple the Hypothesis service URL from the app's base URI

Make these links work when the app is hosted at a different
base URL from the Hypothesis service.

Make these links work when the app is hosted at a different
base URL from the Hypothesis service.

When the Share dialog is hosted in the Chrome extension
and the <base> URI is not set to point to the Hypothesis service,
the share links need to have a scheme set explicitly.

Remove the assumption in the sign-in menu that the
baseURI for the page is the same as the Hypothesis service.

Also use controllerAs and bindToController for consistency
with recently updated directives.

Previously the 'New Groups' link opened '/groups/new' in a new
window, which only worked if <base> URL matched the URL
of the Hypothesis service. Instead explicitly use
the serviceUrl from settings.

Also remove an empty set of tests for the group list controller.
The tests are now all done at the directive level.

Previously the URL set as the <base> tag was used indirectly
to specify the URL of the Hypothesis service that various
links referred to.

Make things clearer by adding an explicit 'serviceUrl' attribute
to the Hypothesis client settings and use that in place of the
baseURI.

The custom template whitelist configuration has been removed
because all of the templates are embedded in app.html, so the
default value of ['self'], allowing templates to be loaded
only from the same domain as the page, is fine.

Fix canceling edits reverting to the first-loaded version of an annotation

Extract the login form into its own directive

This makes the login form consistent with other recently
written components by having the controller and directive in a single
file and using an element directive in the main app.

Only show the 'Create Account' banner in the sidebar

Recognize 'group' as a valid parameter name in search queries

Remove the static blocklist

The configuration option for defining a static blocklist of sites
on which Hypothesis should not load has never been used and
we're not anticipating doing so.

Remove the incomplete Firefox extension

A recent successful prototype of a Firefox extension
based on the new WebExtensions API highlighted that as
the way forwards for Hypothesis on Firefox, as it lets
us offer feature parity with much easier maintenance.

If we decide that we want to support the extension for Firefox pre-45, we may
do that by shimming the Chrome APIs using the JPM SDK APIs, similar
to what https://github.com/einaregilsson/Redirector have done.

This commit removes the legacy incomplete
Firefox extension which is missing all of the recent improvements
and tests for the Chrome extension.