Smarter group activity streams

[seanh]

We already have simple activity streams for groups that have new group or group updated events (see #1631). Smarter/more useful group activity streams would also contain the events for things related to the group: users, datasets, other groups, etc.

This should just mean using a more complex SQL query in the group_activity_list() logic function.

Things might get tricky when a user or dataset joins a group, this may show up as two events (the user or dataset was modified, and the group was modified). We may want to have two separate events for this (one to go in the user or dataset activity stream and one for the group), or maybe we just want one 'user/dataset joined group' event.