Admin OAuth 2.0

This adds OAuth 2 to `admin`.

It has a lot in common (and repetition with #5210); mainly `OAuthLoginController` and the `OAuthCredentials`.

I will put this out before #5210. After the merge of #5210 I plan on generalising `OAuthLoginController` slightly.

This depends on https://github.com/guardian/play-googleauth/pull/7 and https://github.com/guardian/play-googleauth/pull/3.