Allow underscores in domain names

[salim-b]

Expected Behavior

Shortening the URL https://salim_b.gitlab.io/ should just work.

Current Behavior

When trying to shorten the URL https://salim_b.gitlab.io/, e.g. on the demo page, an error message appears:

Error The link URL format is invalid.

Context

Possibly related issues:

• https://github.com/cydrobolt/polr/issues/77
• https://github.com/cydrobolt/polr/issues/514

Some background about _ in URLs on Stack Overflow:

• Can (domain name) subdomains have an underscore “_” in it?
• why use - instead off _ in url