Allow OIDC to support stackspin SSO again

!44 (merged) changes the social login configuration to allow SSO using Gitlab as backend. Unfortunately that has broken stackspin SSO: the userInfoUrl setting is now assumed to contain two path components, both of which are configurable. However, stackspin SSO needs to set it to a path with a single component: /userinfo.

We need to make at least this setting more flexible so both SSO backends can be configured correctly.

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