* 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