For sites that are configured to mute some or all categories and tags
for users by default, groups can now be configured to set members'
notification level to normal from the group manage UI.
* FEATURE: set notification levels when added to a group
This feature allows admins and group owners to define default
category and tag tracking levels that will be applied to user
preferences automatically at the time when users are added to the
group. Users are free to change those preferences afterwards.
When removed from a group, the user's notification preferences aren't
changed.