Add GitHub sign-in for demo.gitea.com

[wxiaoguang]

To help users to test the demo

[wxiaoguang]

@lunny @techknowlogick

[lunny]

Since Gitea.com has supported login with Github/GitLab/Google accounts automatically so that I think it's enough for users to reproduce it.

[techknowlogick]

I've disabled the email/password form on the login page, so now the user is directed entirely to the gitea.com button.

[wxiaoguang]

Since Gitea.com has supported login with Github/GitLab/Google accounts automatically so that I think it's enough for users to reproduce it.

But there are still GitHub users say that "I have no gitea account". When they see "Sign in with Gitea.com", they just go away.

[wxiaoguang]

@techknowlogick @lunny now it's unable to login to demo.gitea.com, I think it's important to make it stable, especially for new users who have interests

[wxiaoguang]

Since Gitea.com has supported login with Github/GitLab/Google accounts automatically so that I think it's enough for users to reproduce it.

But there are still GitHub users say that "I have no gitea account". When they see "Sign in with Gitea.com", they just go away.

@techknowlogick @lunny what's the reason to not support GitHub sign-in for demo.gitea.com?

[lunny]

Since Gitea.com has supported login with Github/GitLab/Google accounts automatically so that I think it's enough for users to reproduce it.

But there are still GitHub users say that "I have no gitea account". When they see "Sign in with Gitea.com", they just go away.

@techknowlogick @lunny what's the reason to not support GitHub sign-in for demo.gitea.com?

Note: GitHub Login will be enabled later.

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[techknowlogick]

I added a (simple js) redirect on the login page for demo so whenever anyone lands on it they will be directed to the gitea.com login page that prompts for GitHub.
I also reinstated the warning banner with regards to repos being temporary.