Change the topic of a previously posted post.

Ticket: [TNL-171](https://openedx.atlassian.net/browse/TNL-171)

**Acceptance criteria:**
Share the Topic Area dropdown from the New Post view to the Edit view of a post.

**sandbox:** http://polesye.m.sandbox.edx.org/

@jimabramson , @jmclaus , @talbs , @explorerleslie please review.

> @polesye Please move into 'utils.coffee' as mentioned in the TODO.

It needs some refactoring that is not a part of this PR. As I found, those methods are duplicated in some classes, so it'd be great to generalise them and move into utils.coffee.