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
both org.thoughtcrime.redphone.util and
org.thoughtcrime.securesms.util have a Base64 util class,
however, other than whitespace changes, and the securesms
one having a couple of extra methods, they are the same.
This commit removes org.thoughtcrime.redphone.util.Base64 and
anything referencing it now uses
org.thoughtcrime.securesms.util.Base64
Closes#4294
