Change the thread type of a previously posted post. [TNL-172]

Ticket: [TNL-172](https://openedx.atlassian.net/browse/TNL-172)

**Acceptance criteria:**
* Share the Post Type dropdown to the New Post view into the Edit view.
* When a question is changed to a post type, all answers should become regular responses (none should be "marked as answered")
* We'll follow this on with a small UX cleanup if there's any UX work needed (no pre-work from the UX team).
 
**sandbox:** http://jmclaus.m.sandbox.edx.org/

**IMPORTANT**: In order for this PR to function correctly, the merge of the following PR in cs_comments_service repository (https://github.com/edx/cs_comments_service/pull/120) is necessary.
This seems totally unnecessary to me. We should make the default value for new posts consistent everywhere.