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

*  `drafts.js`:
  * Update `isDistinct`
* `utils.js`:
  * Add `Utils.multiRecipientMatch` which allows for comparison of multiple recipients without regard to order
* `compose.js`:
  * add `fromDraft` which checks for drafts in a given thread (or conversation) and preloads the draft into the new message composer as needed
* `thread_ui.js`
  * Update the calls to delete or save a draft based on interaction (including sending a message which updates a thread containing a draft
* `message_manager.js`
  * in `launchComposer` add draft recipients and content if a draft is associated with the current conversation
  * For threaded drafts, preload only draft content when entering a thread