Introduce 'group_activity' sort order for group member queries

Adds a new 'Group Activity' option to the 'Order By' dropdown on group member
listings, so that you can see who has most recently been active in that
specific group.

Fixes #3430

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8397 cdf35c40-ae34-48e0-9cc9-0c9da1808c22