Forbid disabled devices from authenticating via SSH or HTTP

Summary:
Ref T13641. Phabricator sometimes makes intracluster requests that authenticate as a device.

Forbid these requests from authenticating as a disabled device.

Test Plan:

• Ran bin/ssh-exec --phabricator-ssh-device ... as an enabled/disabled device (worked; sensible error).
• Made Conduit calls as an enable/disabled device (worked; sensible error).

Maniphest Tasks: T13641

Differential Revision: https://secure.phabricator.com/D21635