Bug 936648 - [Messages][Drafts] Draft display and interaction within a thread

drafts.js:
* Add `Drafts.has(id)` to check if a draft exists in the index

compose.js
* Add `Compose.getDraft(id)` to render a draft for specified id into the message composer
* Not sure yet if compose is the best place for this but code can be moved to message_manager easily if needed

thread_ui.js
* Render draft content when thread is rendered