Nagios checks

[chrismytton]

We should add some extra nagios checks for the following conditions to ensure we get alerted when scheduled tasks don't run correctly:

• New problems that haven't been emailed to the provider within, say, an hour of being created.
• Confirmation emails that haven't been sent out, again within an hour or whatever makes sense.
• Surveys that haven't been sent for closed problems.

[stevenday]

We already have an open issue about this (which I clearly should have closed by now): https://github.com/mysociety/citizenconnect/issues/688 - I'll close this one in favour of that, as that already has a good discussion of my preferred option for implementing this.