Allow Matrix users to send MMS messages. Not entirely sure how
to do this properly, so in the meantime just link the recipient
to the URL of the file on the Matrix server.
Implemented support for MMS messages, including a variety of formats
of image and video. Works by observing changes to content://mms-sms,
parsing out the new MMS contents, uploading them as assets to
the Matrix server, and posting a new image or video message to the room.
