Verify that the homepage URLs are valid and good

[szabgab]

For each released crate check if the URL provided in the "homepage" field is a properly formatted URL to a public web site that is not a "bad" site.
Some invalid values I saw:

• ... (that is 3 dots)
• https:/bla.com single slash
• http://localhost:5000 that's not public

I am not sure how exactly to define what is a "bad" site, but I guess a site with adult content or a site with spam would be considered that. Probably there is some service that can tell us - given a URL if the site is "bad".

See https://rust-digger.code-maven.com/homepages

[carols10cents]

If you identify hosts we should block, please send a PR to add them to this constant. Domains that appear in that list won't have links rendered to them on crates.io pages. Thanks!