Summary:
This diff implements a behavior change described in https://github.com/nylas/N1/issues/1722.
Reply buttons should prefer to focus an existing draft in reply to the same message, if one is pristine, altering it as necessary to switch between reply / reply-all. If no pristine reply is already there, it creates one.
Reply keyboard shortcuts should do the same, but more strictly - the shortcuts should switch between reply / reply-all for an existing draft regardless of whether it's pristine.
This diff also cleans up the DraftStore and moves all the draft creation itself to a new DraftFactory object. This makes it much easier to see what's going on in the DraftStore, and I also refactored away the "newMessageWithContext" method, which was breaking the logic for Reply vs Forward between a bunch of different helper methods and was hard to follow.
Test Plan: They're all wrecked. Will fix after concept is greenlighted
Reviewers: evan, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D2776
