Require 'maintenance' action to keep labellers 'valid'

[Bossett]

Is your feature request related to a problem? Please describe.

If a labeller is abandoned, it is not clear what does/should happen with those labels. If it is actively sunsetted, the service record can be deleted, but an abandoned labeller will have a record that sticks around forever.

Describe the solution you'd like

Labellers should require a maintenance action of some sort to stay 'alive' - this could be an event emitted by ozone, or criteria such as 'emitted label in the last 90 days'. Or the app view could maintain a 'last action' or similar that reflects some activity taken by the labeller.

An 'inactive' labeller should stop applying labels (client should stop sending header) and 'Inactive' should be reflected in the UI on the labeller profile page. Optionally allow an override (by the subscribing user) to allow for one shot/meme labellers.