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.
