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.

> Please remove the link break here and have the <span> and its text node all on one line.

Done.