First cut at group messaging refactor.
Make MMS more asynchronous and consistent with new SMS types.

1) We now delay MMS notifications until a payload is received,
   or there's an error downloading the payload.  This makes
   group messages more consistent.

2) All "text" parts of an MMS are combined into a second text
   record, which is stored in the MMS row directly rather than
   as a distinct part.  This allows for immediate text loading,
   which means there's no chance a ConversationItem will resize.

   To do this, we need to include MMS in the big DB migration
   that's already staged for this application update.  It's also
   an "application-level" migration, because we need the MasterSecret
   to do it.

3) On conversation display, all image-based parts now have their
   thumbnails loaded asynchronously.  This allows for smooth-scrolling.
   The thumbnails are also scaled more accurately.