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 would rather say:
> 
> When the menu is expanded, a click on the body element (outside of the menu) or on a menu element should close the menu except when the target is the search field. To accomplish this, we have to ignore clicks on the search field by stopping the propagation of the event.

Done.