Adapt blacklist implementation to work with complete urls instead of only hosts

[simonfrey]

As it would be nice to add the urlhaus blacklist [see #42) source to unshort.link it would be required to change the blacklist implementation to support complete urls instead of only hosts.

This will make the blacklist check more costly, but would be a nice feature to protect against e.g. viruses on pastebin: https://github.com/simonfrey/unshort.link/blob/master/server/blacklist/blacklist.go#L115-L122

[Madydri]

+1 as phishing sites URL can be inside innocent domains whose owner does not know it is present