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.

it seems like this view is not needed and irrelevant in inline mode - only the tab mode matters.  Instead of always instantiating this view and storing the mode on it, couldn't we instead check the mode in the create / edit post views, and either create it or not accordingly?