Keep the permissions of new annotations in sync with the group, when
changing the focused group.

This fixes an issue where a new (not yet saved to server) shared
annotation in group Foo would become a private annotation if you changed
the focused group to Bar.

The issue is that model/@annotation.group gets changed to Foo, but
model/@annotation.permissions still contains a permissions object with
group Bar's ID in it, and as a result the permissions service thinks
that it is a private annotation.

The fix is to update both .group and .permissions when the focused group
changes, and to update the drafts service _after_ doing this not before
(otherwise a copy of the annotation containing the previous group's ID
gets saved to the drafts service, and then later gets read from the
drafts service and overwrites the correct data).

Fixes #2713.