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.

> I don't think we use *-panel anywhere else in the UI from a naming convention point of view. Since this area contains all of the edit form's UI, I'd recommend using forum-edit-post-form to be descriptive semantically and to avoid the use of *-panel.

Done.