Create an activity item when a group's details are updated.

When a name or description alone are updated, the activity item will display
the "from" and "to" values as part of the formatted activity action string. If
both items have been changed, the activity action will read "x changed the name
and description of the group y".

Fixes #5880.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9204 cdf35c40-ae34-48e0-9cc9-0c9da1808c22