Provide an option to skip login page if OAuth provider is registered

[xathon]

We are using OAuth with our Overleaf server, which causes the "regular" login to stop working. It should be possible to tell Overleaf to get from the login page directly to the OAuth provider.

[yzx9]

Thanks for your PR. I noticed in the PR that when OAuth is enabled, the regular account password login method ceases to function. Is this the intended outcome? I believe this would be better suited as a separate configuration.

[xathon]

That wasn't intended, but it was the behavior I was seeing once I enabled OAuth on the current live code. The regular username/password login only threw an error, so I figured it was disabled by OAuth and wanted to skip it completely. If that's in fact not intended, then this should definitely be a setting, that's true.

[yzx9]

It seems you are indicating that a bug is causing the username/password login to fail only when OAuth2 services are enabled. This behavior is certainly not intended. I have reviewed our current codebase and haven't identified any obvious errors that could be causing this login issue. To address this effectively, could you please create a new issue detailing this specific problem? I will investigate it further.

Regarding the current issue, I understand its significance. However, the proposed Pull Request introduces a BREAKING CHANGE. I would be more inclined to approve it if the change were implemented as an optional feature. This approach would allow users more flexibility in how they integrate this update.