Domains health checks

[acelaya]

Summary

Currently, you can create arbitrary new domains when creating short URLs. However, that doesn't mean those domains will work out of the box, as you still need to configure DNS, web servers, etc. This can be confusing.

It would be useful to add a check, that once the manage domains section is loaded, tries to call the /rest/helth endpoint for each domain. If it "throws" an error, we can display a warning icon with some instructions on how to proceed.

The instructions could be basically a link here https://shlink.io/documentation/advanced/multiple-domains/#external-configuration