Implement replying, post editing, discussion creation

[tobyzerner]

The "composer" is the name of the interface component in Flarum's Ember app which handles all post composition, whether it's writing a reply, starting a discussion, or editing a post. It's been prototyped for demoing purposes, but at the moment is completely unfunctional.

The post and discussion-writing APIs are done and should be working for the most part, but may need to be tweaked.

• Redesign, subject to Interface redesign #1
• Make replying functional
• Make editing posts functional
• Make creating discussions functional