Temporarily disable CAPA problem "Check" button.

- update appropriate jasmine tests so that stubbed $postWithPrefix
respects a subset of jQuery promise API.
- add customization for checking state.
- display checking state while button is disabled.
- ensure checking state lasts for at least a second.
Refactor speed control.
Add fixes.
Update docstrings.
Removed unused helpers.
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
Convert first batch of lettuce video tests to bok-choy.
Merge pull request #2440 from glowe/prevent-duplicate-checks

Temporarily disable CAPA problem "Check" button.
Disable discussion service setting for lettuce tests
Upgrade JSDraw.
Merge pull request #3217 from edx/ammar/convert-lettuce-video-tests-to-bok-choy

Convert lettuce video tests to bok choy
Return 404 instead of 500 for missing forum thread

Also show a more specific error message in the front end. This change
only has an effect if using cs_comments_service commit 1d71330 or later.
Merge pull request #3305 from edx/zoldak/no-lettuce-with-discussion

Disable discussion service setting for lettuce tests
Merge pull request #3307 from edx/gprice/deleted-thread-404

Return 404 instead of 500 for missing forum thread
in Studio, render compact placeholders for inline discussions.

JIRA: FOR-429
Merge pull request #3295 from edx/jsa/disc-preview

in Studio, render compact placeholders for inline discussions.
Support for Duplicate and Delete of XBlock Container Leaves (disabled under Feature Flag).