make sure things have obvious extension points for different platforms

We don't need to *implement* these other platforms but we should make sure we understand how we would add them and don't over-hardcode github things. We should do an audit pass over the endpoints/schemas to make sure there's a reasonable path to add "gitlab" or "bitbucket" or "mozilla's phabricator" as an integration (if only so it's easier for folks to self-host this app with a bit of config).

Potentially relevant extension points off the top of my head:

* Authentication/Login
* Repo/pr "style" (github PR, phabricator PR...)
* Callback/endpoint "style" ("trigger a github action")
* Callback/endpoint url(s) ("my server is at `https://phabricator.mozilla.com/cargo-vet-api`")