Changed default post type to discussion.

Ticket: [TNL-546](https://openedx.atlassian.net/browse/TNL-546)

**Acceptance criteria:**
* When creating a new post, post type should have Discussion selected.
 
**sandbox:** http://jmclaus.m.sandbox.edx.org/

**Note**: The above sandbox has not been rebuilt, it is based on https://github.com/edx/edx-platform/pull/5308 that is still in code review.

@jimabramson @gwprice Please review.