Revert "Support custom authentication backends"

This reverts merge request !334 (merged).

The way custom authentication backends were implemented break too much stuff, or requires workarounds that hide other mistakes. As nothing in AlekSIS uses custom auth backends, and we are currently integratin django-allauth in !413 (merged), I request the feature to be removed.

If someone wants to add a custom authentication backend, they should hook into allauth instead.

@hansegucker Please merge after taking note if you do not have any objective arguments to keep this custom implementation.

Closes #313 (closed)

aleksis/core/tests/test_authentication_backends.py

-from typing import Optional
-
-from django.contrib.auth import authenticate
-from django.contrib.auth.backends import BaseBackend
-from django.contrib.auth.base_user import AbstractBaseUser
-from django.contrib.auth.models import User
-
-import pytest
-
-from aleksis.core.apps import CoreConfig
-from aleksis.core.util.core_helpers import get_site_preferences
-
-pytestmark = pytest.mark.django_db
-
-
-class DummyBackend(BaseBackend):
-    def authenticate(
-        self, request, username: str, password: str, **kwargs
-    ) -> Optional[AbstractBaseUser]:
-        if username == "foo" and password == "baz":
-            return User.objects.get_or_create(username="foo")[0]
-
-
-backend_name = "aleksis.core.tests.test_authentication_backends.DummyBackend"
-
-
-def test_backends_simple(settings):
-
-    assert not authenticate(username="foo", password="baz")
-
-    assert backend_name not in settings.AUTHENTICATION_BACKENDS
-
-    settings.AUTHENTICATION_BACKENDS.append(backend_name)
-    assert backend_name in settings.AUTHENTICATION_BACKENDS
-
-    assert authenticate(username="foo", password="baz")
-
-    settings.AUTHENTICATION_BACKENDS.remove(backend_name)
-
-    assert not authenticate(username="foo", password="baz")
-
-
-def test_backends_with_activation(settings):
-    assert not authenticate(username="foo", password="baz")
-
-    settings.CUSTOM_AUTHENTICATION_BACKENDS.append(backend_name)
-
-    assert backend_name not in get_site_preferences()["auth__backends"]
-    assert backend_name not in settings.AUTHENTICATION_BACKENDS
-    assert not authenticate(username="foo", password="baz")
-
-    print(get_site_preferences()["auth__backends"])
-    print(get_site_preferences()["auth__backends"].append(backend_name))
-
-    get_site_preferences()["auth__backends"] = [backend_name]
-
-    assert backend_name in get_site_preferences()["auth__backends"]
-    assert backend_name in settings.AUTHENTICATION_BACKENDS
-    assert authenticate(username="foo", password="baz")
-
-    get_site_preferences()["auth__backends"] = []
-
-    assert backend_name not in get_site_preferences()["auth__backends"]
-    assert backend_name not in settings.AUTHENTICATION_BACKENDS