We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

If the schema of the database is incompatible with the schema expected
by the code, the database throws a DbSchemaException. LifecycleManager
indicates the error to BriarService, which uses HomeScreenActivity to
show a notification and quit the app.

Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.

This allows quick access to any part of the app, including quitting (two
touches).

This makes the contact list behave how alpha testers expected: touching
a contact shows private messages exchanged with the contact.

Fixes issue #3611924.

Fixes issue #3611925.

(Although it isn't possible to create an identity yet...)

The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.