Add option to make sign-in button link to oauth provider

[tiltingpenguin]

Feature Description

When you only want to let users log into gitea through oauth I think it makes sense to add a setting that allows you to skip the usual login page and automatically link users to the oauth provider.

Essentially I would like to skip the page attached in the screenshot and just go to the configured oauth page

Screenshots

[coolo]

It's noteworthy that we hacked this into https://gitea.opensuse.org as proof of concept - but aren't using a config option.

[techknowlogick]

Thanks @coolo and @tiltingpenguin do you have your patch available to look at?

[tiltingpenguin]

Not yet but I'm working on it right now. I will open a PR if I'm ready