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.

@cahrens It makes me uncomfortable too. It means that there's a mixture of generated and hand-written JS files in that directory. Do all the scripts that clean out generated CoffeeScript JS files no how to handle this? I don't know if they just remove every JS file beneath the coffee directories. Since we're trying to remove CoffeeScript I'd prefer to see us use a js directory.