Web UI: add IndieAuth login

[snarfed]

Right now, we support Bluesky, Mastodon, and Pixelfed on https://fed.brid.gy/login . We should add IndieAuth for web site login! This would let people authenticate their web sites, turn bridging on and off, etc without emailing us and us doing that manually for them. Should be pretty straightforward, oauth-dropins already has IndieAuth support.

Drawback is that IndieAuth isn't widely adopted among web site owners yet, so it might not actually buy us that much. 🤷

cc @anujahooja