Handle un- (or re-)enrollment: clear "ephemeral"-but-saved status report data

[jessepeterson]

Currently we hang on to declaration status, status values, and status errors forever. We should handle the case where a device:

• enrolls
• un-enrolls
• re-enrolls

By e.g. getting rid of all previous errors and values. I wouldn't want to create a web hook handler but I'm not sure of another avenue to get this data over to to us. Perhaps to just start with a simple "reset device" API?

See also #17.