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.
We'll follow this on with a small UX cleanup if there's any UX work needed (no pre-work from the UX team).

`@topic.on('thread:topic_change', @updateGroupDropdown)`