Add group preferences.
Add some simple tests for group prefs.
Fxi Speling
Allow committee members to set group preferences.
Add some tests for setting group preferences.
Ensure admins can set prefs for any group.

Picked up by writing tests.
Flesh out preferences tests.
Add link for admins to edit group prefs.
Send notifications on new group membership requests, and test they arrive.
Don't auto-scroll if the current window contains a fragment.

Previously it would autoscroll even if you were linking to a
specific message. Fixes #242
Merge branch 'group-prefs' into staging