Add INACTIVE state for removed keys

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.