Firebase 2024 signInWithRedirect changes

[MoyTW]

I believe but can't really currently prove that our "Sign in with Google" uses signInWithRedirect, and I believe it's flat-out broken. See:

The result of this is that nobody can sign in with Google. Which seems bad.

Since we do not host using Firebase, we'll need to remediate using one of the following: https://firebase.google.com/docs/auth/web/redirect-best-practices#web_1 or we need to disable to "Sign in with Google" sign in method.

See also #844

[MoyTW]

I haven't tested this with more Google accounts than just, like, my own main account - which already does have an email-based account - so that could be junking it up but as I posted in Discord, that's bad enough!

Ok, I think that:
1) our email-based login works fine
2) our Google-account-based login might be broken in a way that's annoying to fix (details later) but I'm NOT sure about that
3) our Google-account-based login is DEFINITELY hard-broken if you have an email associated with said Google account already in our system

More generally we have an issue that the library we imported to do the signup hasn't been updated in 6 years and that's bad.