Refactor REST API serializers to avoid circular imports

netbox/extras/api/serializers_/customlinks.py

@@ -0,0 +1,26 @@
+from rest_framework import serializers
+
+from core.models import ContentType
+from extras.models import CustomLink
+from netbox.api.fields import ContentTypeField
+from netbox.api.serializers import ValidatedModelSerializer
+
+__all__ = (
+    'CustomLinkSerializer',
+)
+
+
+class CustomLinkSerializer(ValidatedModelSerializer):
+    url = serializers.HyperlinkedIdentityField(view_name='extras-api:customlink-detail')
+    content_types = ContentTypeField(
+        queryset=ContentType.objects.with_feature('custom_links'),
+        many=True
+    )
+
+    class Meta:
+        model = CustomLink
+        fields = [
+            'id', 'url', 'display', 'content_types', 'name', 'enabled', 'link_text', 'link_url', 'weight', 'group_name',
+            'button_class', 'new_window', 'created', 'last_updated',
+        ]
+        brief_fields = ('id', 'url', 'display', 'name')