Eliminate the concept of 'Recipients' (plural). There is now just
a 'Recipient', which contains an Address that is either an individual
or a group ID.
MMS groups now exist as part of the group database, just like push
groups.
// FREEBIE
This was a holdover from Signal's origins as a pure SMS app.
It causes problems, depends on undefined device specific behavior,
and should no longer be necessary now that we have all the
information we need to E164 all numbers.
// FREEBIE
1) Remove all our PDU code and switch to the PDU code from the
klinker library
2) Switch to using the system Lollipop MMS library by default,
and falling back to our own custom library if that fails.
3) Format SMIL differently, using code from klinker instead of
what we've pieced together.
4) Pull per-carrier MMS media constraints from the XML config
files in the klinker library, instead of hardcoding it at 280kb.
Hopefully this is an improvement, but given that MMS is involved,
it will probably make things worse instead.
