Component Monitoring

[jbrooksuk]

This is a big one, but would resolve https://github.com/cachethq/cachet/issues/4425

• Opt components into being monitored.
• Needs some kind of configuration (what if a Header is required to access the URL? Frequency?)
• Add a cron job that runs every minute and checks the actively components.
• Track component availability.
• Dashboard to manage component monitors, track availability etc.
• API