[Triage Metadata] Verify the validity of a triage metadata URL on the frontend

[KyleJu]

When users click on triage button, there should be lint check that verifies the validity of triage metadata URLs and prompt users on the spot if they are invalid. Also See https://github.com/web-platform-tests/wpt-metadata/issues/591.

[foolip]

Would this mean fixing up the URLs automatically when possible?

[KyleJu]

I think I will prompt users when a URL is invalid. For example, showing a text that says Your URL is invalid and user isn't able to traige

[foolip]

Alright, so maybe we can also try to prepend 'https://' and checking if that's a valid URL, so that things like "crbug.com/1234" still work.