Implement device key backups

[kegsay]

Sytests:

    × Can create backup version
    × Can update backup version
    × Responds correctly when backup is empty
    × Can backup keys
    × Can update keys with better versions
    × Will not update keys with worse versions
    × Will not back up to an old backup version
    × Can delete backup
    × Deleted & recreated backups are empty
    × Can create more than 10 backup versions

[anon8675309]

Endpoint is /_matrix/client/unstable/room_keys/version. Server side returns a 404 error. Tested from Element Mobile App on Android.

[kegsay]

#1943
#1945
#1946