Discussion XBlock fixes (part 2)

Background: This PR contains some fixes required to run edX solutions client's systems. Some of those fixes are not yet added to edx/master PR as it's suspended for a little while.

Discussions: most of discussions happened around upstream PR: edx#5565

Partner information: 3rd party-hosted open edX instance, for an edX solutions client.

Merge deadline: before release cuts off.

Issues fixed:
* "Email updates" checkbox was not persisting it's state across page reloads (3-rd party instance only, worked just fine in LMS) - `attr('checkbox')` was used to check checkbox state, which is considered a bad practice since jQuery 1.6. Switched to recommended `prop`
* Creating a post in cohorted environment displayed "visible to everyone", while in reality it was properly created as cohort private post.