Move session state to the Redux store

Fix prevent default override

In current versions of PDF.js, the `RenderingStates` enum is no longer
exported as a global, and the export method appears to be dependent on
the build configuration.

Since `RenderingStates` is a trivial enum, this commit fixes the problem
by just reproducing it in `pdfjs-rendering-states.js`.

Use Node v6.2 on Travis

This keeps things consistent with the version of Node used by h:

    https://github.com/hypothesis/h/commit/cc71f6c571829344e1f50a05b33504a2d42725d4

This fixes an error when trying to load the Hypothesis client on the
stream or standalone annotation pages, caused by both the stream app and
the sidebar app trying to set up a 'Discovery' server for annotation
'guests' to connect to.

With this change, only the sidebar app will set up a Discovery server
and hence it is now possible to annotate the stream and standalone
annotation pages.

Upgrade raven node package

Fix scrolling to direct-linked annotations

Fix adder from affecting website's height on load

The FrameSync service introduced in 55093ebb emitted an
ANNOTATIONS_SYNCED event which unintentionally had a different type for
the `tags` argument than the previous code (`string[]`, instead of
`Array<{tag:string}>`)

This commit updates the code in WidgetController to expect `tags`
to be of type `string[]`.

Fixes #129

Move the session state out of the "session" service and into the Redux
store.

This enables logic in reducers and action creators to access the
logged-in user, list of groups and active feature flags.

Remove crossframe service and simplify communication with connected frames

Now that AnnotationSync is no longer used in the sidebar app, we can
remove all of the handlers for events and messages arriving through the
channel which are not needed by the page itself.

Previously the communication between the sidebar and host frame was
implemented by a shared AnnotationSync class, with a 'crossframe'
service which abstracted the event bus on each side.

This design however assumed that both sides wanted to listen to
the same messages and react to them in similar ways. This is not
the case, especially given the change to use Redux for state
management in the sidebar app.

This commit replaces the crossframe service and AnnotationSync class
with a new 'frameSync' service which implements only the event
listeners and sidebar -> page RPC calls that are actually needed.

As a result, local annotation tags for annotations loaded via the API
can now be assigned by the reducer in the Redux store, making this
easier to test and getting us another step closer to making Annotation
objects immutable in the sidebar app.

Yesterday a brief issue occurred where the server reported an incorrect
(too high) total number of results for a URL due to the search index
being out of date. Once the client reached the end of the actually
available results, it made queries to the search endpoint which returned
an empty result set but had a `total` figure implying that there should
be more pages. The client then ended up polling the server indefinitely
for more results.

This commit defensively makes the client stop fetching more result pages
if the current page is empty.

When calculating the count of pending updates that is used to determine
whether to show the 'Apply Updates' icon or not, count deletions.

In addition to changing the count calculation itself, this also requires
that pending deletions are discarded for annotations that are not
currently loaded. Such notifications may be received because the
deletion may have happened in an un-focused group. Unlike create/update
notifications, no group information is included in the notification
itself so we look up the ID in the local store.

Update icon for applying updates to the current page

As per discussion at
https://hypothes-is.slack.com/archives/frontend/p1474276933000039,
adjust the wording of the 'Apply Update' icon tooltip to avoid "update"
being misinterpreted as referring to a software update.

Update icon to match current design from https://trello.com/c/tRZ2H7iH
and remove the update count, as per the most recent comments on the
Trello card.

The deferred update UI was added primarily to avoid the problem of
annotation updates being distracting while a user is composing
annotations in the sidebar. This is less relevant on the stream.

Additionally fix a bug where updates from unfocused groups were not
shown on the stream, since the stream shows updates from all groups.

Since we discard all pending updates and deletions when switching
groups, discard create/update notifications about annotations in
unfocused groups as soon as we receive them.

This fixes the issue where the 'Apply Update' badge counted updates
from other groups, with the result that clicking the badge did not
show any changes.

Move the logic that updates the focused group for new annotations
out of the <annotation> component and into a service.

This fixes the issue where the event handler does not get invoked if no
<annotation> component instance exists for an annotation because it is
currently scrolled off-screen.

Fixes #105

Previously when switching groups all of the currently loaded annotations
would be unloaded and then potentially outdated copies of the annotation
from the drafts store would be reloaded into the annotation store.

This commit fixes that problem by:

 1. Only unloading saved annotations when switching groups.

 2. Only storing the annotation ID and tag in the drafts service, to
    ensure that we don't try to use it as a complete annotation anywhere

A consequence of (1) is that new, unsaved annotations no-longer briefly
disappear when switching groups.

This fixes the problem described in #109 where the orphan status of a
new annotation is lost when switching groups. It does not fix the
problem where the new annotation immediately becomes an orphan.

The logic to remove new and empty annotations used to live in the
<annotation> component because that was the only part of the code that
had access to the state of unsaved changes.

Since <annotation> instances are only created for on-screen annotations,
this can result in empty drafts not being removed if the empty
annotation is off-screen.

Now that the canonical content of unsaved annotations is stored in the
drafts service, we can move the logic outside of the annotation
component and fix this problem.

Fixes #97

Defer realtime update