Application was always picking first participant when tapping on avatars
in conversations. It was also using first participant for MMS notifications.
This stores sender's phone number in the database, so it can be used to look
up correct participant in the list of participants. If matching on number fails,
matching on name is attempted. If both of these fail, it falls back to previous
behavior, which is just picking the first participant.
This may also be connected to #32, but I am not sure, since this should just
be related to behavior when tapping on avatars. Mixing up avatars in the
conversation should be a different issue.
This closes#433, closes#500, closes#384
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