Query ContentTypes instead of distinct LogEntry

SELECT DISTINCT app_label, model FROM log_entry is a very expensive
request for longer logs, while we can always get the list of all
tracked models straight from the registry.

This new approach has two downsides:
1. It only provides filters for currently tracked models.
2. It can list such filter options that don't have any log entries.

src/auditlog/filters.py

@@ -1,4 +1,9 @@
 from django.contrib.admin import SimpleListFilter
+from django.contrib.contenttypes.models import ContentType
+from django.db.models import Value
+from django.db.models.functions import Concat
+
+from auditlog.registry import auditlog
 
 
 class ResourceTypeFilter(SimpleListFilter):
@@ -6,9 +11,17 @@ class ResourceTypeFilter(SimpleListFilter):
     parameter_name = 'resource_type'
 
     def lookups(self, request, model_admin):
-        qs = model_admin.get_queryset(request)
-        types = qs.values_list('content_type_id', 'content_type__model')
-        return list(types.order_by('content_type__model').distinct())
+        tracked_model_names = [
+            '{}.{}'.format(m._meta.app_label, m._meta.model_name)
+            for m in auditlog.list()
+        ]
+        model_name_concat = Concat('app_label', Value('.'), 'model')
+        content_types = ContentType.objects.annotate(
+            model_name=model_name_concat,
+        ).filter(
+            model_name__in=tracked_model_names,
+        )
+        return content_types.order_by('model_name').values_list('id', 'model_name')
 
     def queryset(self, request, queryset):
         if self.value() is None:

src/auditlog/registry.py

@@ -114,6 +114,10 @@ class AuditlogModelRegistry(object):
             'mapping_fields': self._registry[model]['mapping_fields'],
         }
 
+    def list(self):
+        """Get a list of all registered models."""
+        return list(self._registry.keys())
+
 
 class AuditLogModelRegistry(AuditlogModelRegistry):
     def __init__(self, *args, **kwargs):