Activity: Option to sort by last_modified datetime (for child comment replies)

[nuprn1]

forgive me if i'm missing a way to do this:
http://buddypress.org/forums/topic/request-new-reply-to-bring-thread-up-in-activity-directory

A database change for a overall last_modified datetime so a SORT can be passed in. (last_modified is updated whenever that activity parent is commented on) This way - activity stream items which are being commented on can be brought to the top and the discussion not drop off.

[boonebgorges]

I think this is probably going to happen for 1.3. Duplicate of #2204