We introduce a new trust state: INACTIVE. This state is intended for
old keys that have been removed.
When a TRUSTED device is removed from the PEP devicelist, it's status
will be set to INACTIVE. INACTIVE keys are shown in the UI as greyed
out, non-interactible key rows. Messages are not encrypted for INACTIVE
devices.
When an INACTIVE device reappears in PEP, or a message is received from
an INACTIVE device, it is set back to trusted.
If the contact (or the own account) has keys that have UNDECIDED trust,
we now drop the user into the new TrustKeysActivity, where they have to
decide for each new key whether it should be TRUSTED or UNTRUSTED.
