Integrate an OAuth2 provider

[tboerger]

To make it easier for other applications to hook into Gitea we should integrate an OAuth2 provider, that way tools like Drone CI can authenticate against Gitea much easier. A good library for that can be https://github.com/RangelReale/osin.

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[joubertredrat]

Oh, sounds good this :)

[bkcsoft]

Should this be integrated as "The" login-handler, or as an optional dependency? (i.e. build tag)

[tboerger]

I think we can always integrate it but add an option for admins to disable it

[lunny]

No build tag but default is closed until admin open it.

[joubertredrat]

Nice idea 👍

[bkcsoft]

@tboerger @lunny I was more wondering if all Authentication should be handled by OAuth, therefore removing the old auth-module