feat: OAuth

[iuioiua]

This change adds OAuth functionality for authentication. So far, GitHub is used as an example.

I'm aware that this approach is suboptimal compared to the incoming PKCE approach but would rather enable OAuth sooner rather than later. We'll switch to PKCE once available.

Towards #13.

@thorwebdev and @kt3k , WDYT?

[thorwebdev]

Thanks @iuioiua , and sorry for the delay on PCKE. We needed to fix some issues in gotrue-js but those have now been released. Hoping we'll land PCKE support in the auth helpers by end of week!

[iuioiua]

Thanks @iuioiua , and sorry for the delay on PCKE. We needed to fix some issues in gotrue-js but those have now been released. Hoping we'll land PCKE support in the auth helpers by end of week!

Not a problem! One of the reasons I made this PR was so that we can directly see the improvement once PKCE has landed.

[iuioiua]

@thorwebdev, does the implementation look sound to you? It was taken from https://github.com/thorwebdev/everydaylytics.

[thorwebdev]

Yes, LGTM 👍

[kt3k]

Other part looks good to me.

[iuioiua]

Changes made:

1. Login/signup pages look prettier.
2. Changed /login-success route file to /login/success and /login to /login/index.tsx.
3. If provider isn't a string, return HTTP code 400.