Update OAuth bindings / plugins

[Cervator]

The login options via OAuth on the forum haven't been maintained for ages and it shows, for instance:

http://i.imgur.com/W3PRNSF.png

(bad labels)

Somebody reported one of them not working at all (Twitter?) some time ago, and there are more options available we could set up, like Steam. Wonder if there is a GitHub one yet ...

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[Cervator]

The FB app used to link the forum is showing an expiration warning now (deprecated API, getting retired)