Support logging in with an OAuth2 provider

A rephrasing of the now-closed #558. And a reincarnation of #39.

As a user, I would like to be able to log in with a 3rd-party identity provider and not have to maintain yet another username and password credential pair.

This is probably only going to be useful for self-hosted installations -- they can decide who they trust. For the publicly hosted version, I don't know of many OAuth2 providers that has broad community trust. (Mozilla's Firefox Accounts maybe? I don't even know if 3rd parties can use it. Gitlab and/or Github as well?)

If support is implemented in a generic fashion, then users can spin up their own OAuth2 providers (RedHat's KeyCloak, a self-hosted Gitlab instance, there are many other projects.) and trust those.