Migrate to supporting Uuid over e164

Signal is moving over to Uuid as a base of identity, instead of phone numbers.

• Tolerate storing identities and sessions identified through UUID (75211426, part of !37 (merged))
• Fetch user profile and force-migrate sessions to UUID. #192 (closed)
• Support UUID based source in database. !101 (merged)
• Show local UUID in Settings page 23b2eeb5

The following discussions from !101 (merged) should be addressed:

• @rubdos started a discussion:

  Make sure that peer identity can be computed from UUID alone. Peer identity needs to be able to switch from e164 to UUID. -> #303 (closed)

• @rubdos started a discussion:

  GroupDetails synchronisation: send more data (UUID ! #80 (closed)). -> #275