Create a ActivitypubResponse class and use it in the views

This improves compatibility with other fediverse software by using the
expected content type for any api calls.

bookwyrm/activitypub/response.py

@@ -0,0 +1,18 @@
+from django.http import JsonResponse
+
+from .base_activity import ActivityEncoder
+
+class ActivitypubResponse(JsonResponse):
+    """
+    A class to be used in any place that's serializing responses for
+    Activitypub enabled clients. Uses JsonResponse under the hood, but already
+    configures some stuff beforehand. Made to be a drop-in replacement of
+    JsonResponse.
+    """
+    def __init__(self, data, encoder=ActivityEncoder, safe=True,
+                 json_dumps_params=None, **kwargs):
+
+        if 'content_type' not in kwargs:
+            kwargs['content_type'] = 'application/activity+json'
+
+        super().__init__(data, encoder, safe, json_dumps_params, **kwargs)