More speedup for the team activity pages

Split the activity into 2 categories:
  - "videos" most activity on team videos
  - "team" members joining/leaving and team video deletion

Having to only select 1 or the other makes the queries much faster.
Also speed up the team videos query substantially with an extra()
call.
Merge pull request #1798 from pculture/speedup-team-activity2

More speedup for the team activity pages