Add healthchecks.io integration

[Hamster45105]

Healthchecks.io is an online service for monitoring regularly running tasks. Instead of directly implementing SMTP notifications for downtime (#214), an integration to this service would be easier to make and maintain. Healthchecks.io has a generous free plan of monitoring 20 jobs so no one would have to pay to set it up. It is also an open source project.

UI

All that would need to be set up in the UI is a textbox on the settings page, where people could input the UUID of their check. Basic instructions on how to set the check up on healthchecks.io would also be required.

Backend

Every five minutes (or 10 minutes or 2 minutes, whatever is suitable), the feeder would have to ping https://hc-ping.com/<uuid> to let the service know it is still alive. If there are any problems with the feeder (eg. a feed is down or any other errors the app catches) the feeder would need to ping https://hc-ping.com/<uuid>/fail and not do any more of the recurring pings until the problem is resolved when it could then ping https://hc-ping.com/<uuid> again. The full docs for the API are located here.

Obviously, this could be extended a lot such as adding options for customising the time between pings, add seperate checks for different feeders but I think this would be the simplest implementation.