Reports for maintenace

[JeanFred]

As a Cabot manager, I would like

• a list of deactivated checks,
• a list of failing instances
• a list of checks attached to no services

so I can prune the obsolete ones and reactivate the meaningful ones

[MrSaints]

Related #530

[JeanFred]

I just realised that the views allow you to sort by status and service, which is an acceptable workaround for these use cases.