Health check limits

[andymckay]

Allow health checks to choose what services they want to run against.

It could be:

• All (what it was before)
• Some (allow you to chose what to include)
• None (allow you to run a health check without any service attached)

The case for some is pretty straightforward, it avoids a health check result and Action being triggered on the server. Saving everyone some time.

The case for none is less straightforward, but you could have a health check unrelated to a service. For example, does everyone in the organisation have MFA enabled? In this case we need to make health check payload and results have service as optional.

To do:

• update docs
• an end to end test with an action

Backwards incompatible change: the command line management tasks were useful at the beginning but after moving to celery seemed pointless. Also there's a button in the UI so I'm not sure what purpose they served other than making code a bit more complicated, so I removed them.