thanks for the tests, i know they took work. i appreciate it!!!

here are a few other test cases it'd be good to have eventually. TODOs are fine, you're welcome to add them after this PR.
* [ ] rel-feed link
* [ ] no h-entries
* [ ] existing SyndicatedPosts
* [ ] activity with existing responses, make sure they're merged right
* [ ] not a webmention target
* [ ] failed domain_url fetch
* [ ] failed post permalink fetch
* [ ] no author url