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.
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
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).
fix for fuzzy prompt on container page
Merge pull request #3234 from edx/nimisha/container-duplicate-delete

Nimisha/container duplicate delete