Create GoToSocial backend

[simcop2387]

We'd love a Perl based backend but that's not ready at the moment

GoToSocial looks to be a really nice backend that exists now.

Key Features

1. Mastodon API compatibility, allows for many existing clients to work (Tusky, Pinafore, etc)
2. Supports OIDC for auth, which allows Keycloak and others
3. Fully Open source

[perigrin]

GoToSocial has a few rough edges and not everything is 100% compatible with the current Mastodon APIs: e.g. I've had a few clients utterly fail to login via native OAuth, and some of the client features are 404s … and video support is still an open PR while they sort out how they want to implement it.

This isn't suggestions against it, I'm running it myself for nerdfight.online.