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 depenecdy footprint and sets us up for further improvements that are planned upstream. Those will reduce the dependency footprint even more and close #4002

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-adjencent code that can probably be removed / rewritten.

Closes #6281

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 password-reset flow
• implement email-verification flow
• re-implement registration flow
• re-implement password reset flow
• 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]

@sur5r this re-implements the MFA stack and now keeps the user completely in PUI. It is not fully done but I would appreciate if you could test this and report back. Please do this on a test-database, we are migrating the factors to a new DB schema so there could be issues.
Registering factors is not working currently but will be implemented before the merge.

[sur5r]

Will do, but might take some days before I get around to it.

@matmair could pass by at PoC desk if you have time?

[matmair]

@sur5r which PoC desk?

[sur5r]

At 38c3, first floor. https://38c3.c3nav.de/l/c:1:193.66:143.06/