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

drafts_test.js:
* Update storage tests to include objects (for MMS drafts)

compose.js
* Add `Compose.fromDraft(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
* update mock

thread_ui.js
* Render draft content when thread is rendered