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
Merge remote-tracking branch 'origin/release' into merge-release-into-master
Merge pull request #3297 from edx/merge-release-into-master

Merge release into master