Speedup team activity page

The slowest part of that page was counting all the action objects and
that wasn't really needed.  Replaced the paginator with a button that
loads the next 20 actions.

I tested the activity page on staging and a demo instance of the speedup-team-activity branch.  
Querying against the TED activity page the difference was 50-60 seconds / page vs 2 seconds 
/ page on the speedup branch.  @ehazlett - maybe there's other differences between staging and 
a demo branch?