Support for multiple login provider

[torgeirl]

Trix's OpenID Connect (OIDC) support is focused around one provider (Dataporten), but it would be good to make it provider agnostic so that we can either switch to another provider if-need-be or run multiple providers simultaneously.

We should:

• remove hard references to Dataporten as login provider in the code ✔️
• move login provider config from SocialApp (Django Admin) to settings.py ✔️
• allow multiple login providers in parallel ✔️
• when more than one login provider is available, the users should be asked to choose ✔️
• display the name of current login provider (if any) next to last login details on /user/ ✔️

TO-DOs

• remove or fix (link back to origin?) burger menu on login provider selection page
• find a less hacky solution for adding logout URLs and expected responses
• translations for login provider selection page (Translations for login provider selection #158)

Known issues

• if you log in using an Allauth OpenID Connect provider you have used before, but with another login backend offered by that identify provider (IdP) you get stuck at /authenticate/allauth/3rdparty/signup/. I suspect the issue is that the Django user object is already associated with a SocialAccount from that provider, but that the response can't be overwritten the SocialAccount from the provider's other login backend.