Previously if the user visited a `/a/<ID>` URL and the request to fetch
the annotation failed with a 404 then the user would see a blank page.
If the fetch failed because the annotation was private, logging in did
not cause the annotation to be fetched.

This commit resolves the two issues:

 - When the logged-in user changes, re-fetch the annotation
 - If the annotation fetch fails, an "Annotation unavailable" message is
   shown. This currently repurposes the error that is shown when a
   direct-linked annotation fetch fails, but without the "Clear
   selection" button.

This commit also fixes an issue where a direct link to a group that
could not be fetched would show a "Clear selection" button even though
there was no active selection to clear. That button is only needed when
following a direct link to an annotation.