Merge pull request #4355 from edx/jsa/new-post-style

Update forum new post form
Handle question posts from comments service
Clean up dead forums front-end code
Unify forum thread views (and templates)
Distinguish forum thread types in rendering

Each thread's type is now included in its post details, and question
threads have marked answers shown immediately below the body of the
initial post.
Add endorsement info to marked answers in forum
Change endorsement UI for forum question threads
Re-render forum responses when marked as answer