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.

> Please use dot notation: entries: this.renderCategoryMap(map.subcategories.name)

I cannot use it here, because `map.subcategories[name]` and `map.subcategories.name` is not the same. It is not an attribute `name`.