make cards clickable

[glowinthedark]

It's a frequent use case to need to navigate to a service rather than just displaying its status. This will also allow using the status page as some kind of 'dashboard' that allows navigating to services.

For entries of type: ping the url could, for example, point to some kind of configuration page or whatever the user finds appropriate.

The PR adds the url key to checks.yaml which can point, for example, to the service main page or some custom troubleshooting URL. If the key is missing then the value of the host key is used as a fallback.

Example:

    - name: Wikipedia Home
      type: http
      host: https://www.wikipedia.org
      url: https://en.wikipedia.org/wiki/Main_Page
      expected_code: 200

[harsxv]

Thanks for the clickable cards! Thumbs up.
However, I’m not a fan of the blue color for visited text. Could we please change it back to black?

Also, it might be useful to make this optional, so if no url is defined, the card remains static and non clickable.

[glowinthedark]

@harsxv: removed fallback to host when there is no url in the config; also restored black shades

[harsxv]

Perfect! Thank you