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.
Merge pull request #2440 from glowe/prevent-duplicate-checks

Temporarily disable CAPA problem "Check" button.