Links starting with mailto: are treated as malformed URLs when they (usually) are not

[stevejalim]

It would make sense to add some parsing/checking of mailto: links and then add them to the allowlist, rather than just raise an issue as if it's an actual malformed URL.

Django's URL validation (which is what we use) doesn't seem to treat mailto:[email protected] as a valid URL, so we'll have to write our own, which is fine