Allow multiple 3rd party connections

[SoftCreatR]

This has been suggested already for WCF 2, but there's still no solution.

While almost every website allows you to register/log in using multiple 3rd party providers (if they even provide this functionality), WoltLab is the only one I know, that supports just one connection at a time. I don't see a reason, not to support connecting with multiple providers at the same time, not even technically.

So for a future release, I would like to have the possibility to connect to multiple 3rd party providers, instead of being limited to just one.