add "Follow group" option

[jdalsem]

This feature should do the following:

• add a plugin setting to enable this feature
• add a "Follow group" button to a group profile page / status box (you can only follow groups that have public content enabled?)
• add "groups i follow" filter to widgets / tabs
• you get content creation notifications for groups you follow
• you can get digests / newsletters ???
• special activity page filter ??
• amount of followers visible on group profile (where you can also see members)
• you can not be a member and a follower at the same time