When the client receives a notification that the
list of groups changes, update just the groups list
and, if necessary, focused group rather than reloading
the whole view.

This is done by splitting the SESSION_CHANGED event
into finer-grained GROUPS_CHANGED and USER_CHANGED
events. The <groups-list> directive now listens for
GROUPS_CHANGED and updates itself in response.

This fixes an issue where joining or leaving a group
would always result in unsaved changes to annotation text
being lost.

If the user leaves a group which is currently focused and
which has an unsaved changes to an annotation, that will still
result in changes to the annotation being lost.

 * Add finer-grained GROUPS_CHANGED and USER_CHANGED
   events which components can react to.

 * Avoid directly exposing the groups service to the
   <group-list> template and instead only expose
   the required methods. This makes it easier to
   track what is going on in the template.

Fixes #2641