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.

Underscore templates are stored in the [mako template](https://github.com/edx/edx-platform/blob/00c7e60e07863aaa69da171925202f48ab0b69b5/lms/templates/discussion/_underscore_templates.html) and both, mako and underscore, variables are used here. So, I'm not sure if it's possible right know.