membershipworks: Use EXISTS for testing flags in is_active

This behaves much more correctly, although may not have ideal performance

membershipworks/models.py

@@ -3,7 +3,7 @@ from typing import Optional
 import django.core.mail.message
 from django.conf import settings
 from django.db import models
-from django.db.models import Q, Count, BooleanField
+from django.db.models import Exists, OuterRef
 
 
 class Flag(models.Model):
@@ -21,14 +21,17 @@ class Flag(models.Model):
 
 
 class MemberQuerySet(models.QuerySet):
+    # TODO: maybe rename to reflect EXISTS?
     @staticmethod
     def has_flag(flag_type: str, flag_name: str):
-        return Count(
-            "uid",
-            filter=Q(flags__name=flag_name, flags__type=flag_type),
-            output_field=BooleanField(),
-        )
+        return Exists(Flag.objects.filter(
+            type=flag_type,
+            name=flag_name,
+            members=OuterRef("pk")
+        ))
 
+    # TODO: it should be fairly easy to reduce the number of EXISTS by
+    # merging the ORed flags
     def with_is_active(self):
         return self.annotate(
             is_active=(