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.
@gwprice
> This shouldn't be necessary given that the thread view is reloaded

Do you mean the ```this.trigger('comment:endorse');```?

Without it, if you have a question that has been answered and then switch its type to discussion then back to question, all the responses will be marked as unanswered but the side bar will still have a green checkmark instead of a red question mark.