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.

> Again, I would much prefer that a separate directory structure be created for JS files instead of putting them within a coffee directory.
> 
> In CMS and LMS, we have both coffee and JS directories

Yes, I knew that. But I would like to see all the files in the same directory and do not like to find them in different places of the project.