include redirect targets in duplicate checking

[sknebel]

duplicate submission happened right now with https://petermolnar.net/internet-emotional-core and https://petermolnar.net/internet-emotional-core/, despite the former being submitted later and redirecting with 301 to the latter

[fluffy-critter]

A similar-but-opposite thing happened recently, with https://beesbuzz.biz/articles/3951-Twitter-alternatives being submitted, then having its URL updated to https://beesbuzz.biz/blog/3951-Twitter-alternatives; my webmention sender should have sent an update to indienews where the /articles/ version now redirects to the /blog/ version. This is a much weirder corner case to handle though.