Change behavior of cohort addition interface

Instead of noting users that are already in a cohort and not changing
such users, clobber the previous cohort and display a message indicating
the previous cohort membership.

JIRA: FOR-452
Merge pull request #3282 from edx/hotfix/2014-04-09

Hotfix 2014-04-09
Update video
Use bulk email as is for course email
Remove git version from STATIC_URL
Disable cache busting
Disable cache busting for studio as well
Add XForwardedFor middleware
Change redirect URL when disconnected
Change logo url when not logged in
Add create_account server events to tracking log