Bug 935177 - [Messages][Drafts] Update "discard message" confirmation prompt to options menu/dialog; add "save draft" no-op r=julienw

https://bugzilla.mozilla.org/show_bug.cgi?id=935177

- New strings:
  - save-as-draft      = Save as Draft
  - discard-message = Discard
- Removes window.confirm mechanism
- Implements Multi-option prompt with OptionMenu
- Basic tests to assert the correct operations are performed by the option handlers

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Bug 936641 - [Messages][Drafts] Create message drafts upon user interaction
Bug 936648 - [Messages][Drafts] Draft display and interaction within a thread