Prevent regular login in backend when it's disabled

This rejects any regular login attempt when the regular login form has
been disabled using DISABLE_REGULAR_LOGIN.

Reverts fa06b71517db3e9c8959df076ac432dc6bb4674c, we already have a
setting that manages regular signups.

src/paperless/adapter.py

@@ -2,6 +2,7 @@ from allauth.account.adapter import DefaultAccountAdapter
 from allauth.core import context
 from allauth.socialaccount.adapter import DefaultSocialAccountAdapter
 from django.conf import settings
+from django.forms import ValidationError
 from django.urls import reverse
 
 
@@ -9,10 +10,13 @@ class CustomAccountAdapter(DefaultAccountAdapter):
     def is_open_for_signup(self, request):
         allow_signups = super().is_open_for_signup(request)
         # Override with setting, otherwise default to super.
-        return (
-            getattr(settings, "ACCOUNT_ALLOW_SIGNUPS", allow_signups)
-            and not settings.DISABLE_REGULAR_LOGIN
-        )
+        return getattr(settings, "ACCOUNT_ALLOW_SIGNUPS", allow_signups)
+
+    def pre_authenticate(self, request, **credentials):
+        if settings.DISABLE_REGULAR_LOGIN:
+            raise ValidationError("Regular login is disabled")
+
+        return super().pre_authenticate(request, **credentials)
 
     def is_safe_url(self, url):
         # see https://github.com/paperless-ngx/paperless-ngx/issues/5780