Remove initial redirect when sidebar loads

Avoid redirecting to '/viewer' when the sidebar loads.

This redirect is unnecessary since the sidebar never changes its mode
once loaded and it caused the viewer embedded on a page to fail to load
when restoring a closed tab in Chrome.

This may also resolve issues where the user agent disallows the
'replaceState' call in certain situations (search prod-client reports on
Sentry for "replaceState")

Fixes #178

Update tag service urls

Show annotations in Annotations tab but with quote struck through after anchoring timeout

Since we're not linking to the stream anymore.

Instead of marking annotations as orphans after the anchoring timeout
expires, instead display them in the Annotations tab but with a visual
indicator that they have not yet anchored. This avoids showing
annotations that are still anchoring in the Orphans tab, which can be
confusing when it happens in long PDF documents with many annotations.

 * Add an `$anchorTimeout` flag to annotations that is set if
   annotations fail to anchor within the timeout period.

 * Display annotations with this flag set in the Annotations tab,
   on the optimistic basis that they will eventually anchor.

 * When rendering annotations with this flag set, display them with
   a quote struck through (same as orphans) to indicate that they are
   pending. The visual representation of the pending state may be
   tweaked in future.

Consolidate logic for annotation <-> tab assignment

Move the list of connected frames to the Redux store

Use a custom element name for highlight spans

Previously the logic for determining which tab an annotation should appear
in occurred both in `root-thread.js` where it was used to filter
annotations based on the selected tab, and in `widget-controller.js` where
it was used to switch to the tab containing a particular annotation.

Consolidate all the logic for deciding which tab an annotation should
appear in into a single module. This makes it easier to test that tab
assignment is correct with different combinations of feature flags and
annotation states and to add additional logic in future.

Move metadata about the frames that are connected to the sidebar app,
such as the document's URL and fingerprint, to the central app state
store.

This is part of an effort to unify how important application state is
managed.

Add HTML baseline tests

Add a set of baseline tests for HTML anchoring which attempt to anchor a
set of selectors for an annotation using data in the format returned by
the Hypothesis API and then verify that the same selectors are generated
when re-describing the anchored ranges.

Remove 'defer_realtime_updates' feature flagging

This is now enabled by for everyone, so we don't need the feature flag
any more.

When anchoring many annotations on a PDF, the text of the same page may
be requested many times.

The text is initially fetched via an async call to
PDFViewerApplication.pdfViewer.getPageTextContent() and the result is
then cached for future use.

Previously the cache was only updated once this expensive call
returned a result. Consequently when anchoring many annotations at once,
there would be a lot of unnecessary cache misses and repeated calls to
PDFViewerApplication.pdfViewer.getPageTextContent()

Fix the problem by storing a promise for the result in the cache when
the text for a given page is requested the first time. Consequently the
text for each page will only be fetched once.

Use the same trick that was applied to the adder in
d9644a80 to reduce the likelihood of the
page's own CSS styling modifying the appearance of highlights.

Compared to alternative methods which would raise the precedence of
highlight styling (inline styles, !important !all !the !things, selector
precedence hacks), this approach is simpler as it makes the conflicting
rule from the page's own styling not apply at all.

Fixes hypothesis/h#3520

Added links to help center from help panel

Fix Jenkins builds failing if package.json or shrinkwrap change after branch is initially pushed

This fixes a couple of incorrect usages of Sinon assertions which
resulted in test failures with the latest patch version of Sinon.

 * assert.calledOnce() only takes one argument

 * Sinon assertions do not take a message argument as Chai assertions
   do. See http://sinonjs.org/docs/#assertions

* Move event handlers out of AnnotationUISync

As a step towards having all bridge event handlers in one place, move
these event handlers into frameSync.

 * Add tests for findIDsForTags() and for re-exporting of this
   function from annotationUI

* Move sidebarOpened event handler to FrameSync service

* Remove the AnnotationUISync service

Its functionality has now been moved into the FrameSync service, except
for the logic that was intended to synchronize the "Show Annotation
Highlights" logic across multiple frames, since this is clearly broken
[1].

The two pieces of functionality related to this we need to support at
the moment are:

 1. The `showHighlights` config option to set whether highlights are
    initially visible.

 2. The highlight toggle button in the sidebar's outer frame

Both of these work without the sidebar app's involvement.

[1] See https://github.com/hypothesis/h/issues/3433 and
    https://github.com/hypothesis/h/pull/3295

Jenkins is currently configured to re-use workspaces between builds of
the same branch. Run `make clean` at the start of the build to ensure
that any artefacts from the previous build of the branch are removed.

With the following steps, the second invocation of `make` should trigger
installation of npm dependencies:

 1. Run `make`
 2. Edit dependencies in `package.json`
 3. Run `make` again

The shrinkwrap was not updated when `requests` was updated from v2.71.0
to v2.72.0.

Fix normalization of ranges that start at an element node with no children

This isn't needed since we're not translating error messages.

Rename the `$$tag` annotation property to `$tag`.

The range normalization code failed to take account of ranges where the
start container was an empty element with no children.

Fixes #73

Replace the Range functions from the compiled version of Annotator 1.2.x
with the latest CoffeeScript source from the v1.2.x branch of Annotator,
plus the fix added in b3056d27

This includes fixes for BrowserRange#normalize from
https://github.com/openannotation/annotator/commit/9bec776511a717ef944b36ed7cca3d107bd95c88

In addition, this removes the dependency on the Annotator framework from
annotator/anchoring and the highlighter, getting us closer to a point
where we can remove the dependency of the annotation layer on Annotator.

Fixes #109

Adding code coverage reporting to Karma tests