Remove django-allauth-2fa, dj-rest-auth and django-user-sessions

[matmair]

This PR remove django-allauth-2fa and replaces it with the inline functions of django-allauth. This reduces the dependency footprint and eliminates a bunch of custom creations.

Update: Most of the planned things we would be interested in are now implemented and the scope shifted a bit - it makes sense to just ripp all auth extensions out and use only allauths built-in things. We have a lot of grown auth-adjacent code that can probably be removed/rewritten.

Closes #6281
Closes #8708

Todo:

• refactor authentication forms && detection of enabled features (see 8912)
• implement custom link target lookup for email from django-allauth
• show user ID in MFA flow
• implement email verification flow
• re-implement registration flow
• re-implement password reset flow
• re-implement password change
• keep auth context somewhere for the local session to refer back to
• move now removed system status info into main status (registration, mfa, sso available)
• switch back to browser flow for auth APIs
• rename all new auth URLs to fix to the current schema
• include schema for auth with central API
• write changelog

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[matmair]

@SchrodingersGat @wolflu05 this is getting close; if you have time I would appreciate feedback on this already

[SchrodingersGat]

@matmair this looks like a fantastic improvement. I really appreciate that we can remove a lot of custom auth functionality here.

Have you performed any SSO testing as part of this? Is there any way that we can implement some automated CI testing of SSO auth somehow?

[matmair]

@SchrodingersGat I hope to get most if not all our custom things upstreamed - I am looking into contributing a few different things that we have built for ourselves up to django-allauth, send my first suggestion regarding this a few hours ago. This will help with better-separating concerns between different components.

I have tested SSO with GitHub and a few different OIDC providers extensively throughout working on this. We are not modifying the logic around this in any way so allauths tests should cover it. Adding CI testing would probably entail staging credentials for some provider into the CI - I am instinctively not a fan regarding this. I have not looked into that in more depth because we are just consuming the APIs.

[matmair]

All flows are now ported, now it is about cleanup and finishing a few smaller todos