Always prefer shared inboxes when computing receipent lists

This avoids duplicate submissions to remote instances when mentioning
followers (i.e., `POST /user/foo/inbox` followed by `POST /inbox`, which
results in two separate `add_status` tasks, and might generate duplicates
in the target instance).

bookwyrm/models/activitypub_mixin.py

@@ -153,7 +153,7 @@ def get_recipients(self, software=None) -> list[str]:
         mentions = self.recipients if hasattr(self, "recipients") else []
 
         # we always send activities to explicitly mentioned users' inboxes
-        recipients = [u.inbox for u in mentions or [] if not u.local]
+        recipients = [u.shared_inbox or u.inbox for u in mentions if not u.local]
 
         # unless it's a dm, all the followers should receive the activity
         if privacy != "direct":

bookwyrm/tests/models/test_activitypub_mixin.py

@@ -227,14 +227,18 @@ def test_get_recipients_combine_inboxes(self, *_):
                 shared_inbox="http://example.com/inbox",
                 outbox="https://example.com/users/nutria/outbox",
             )
-        MockSelf = namedtuple("Self", ("privacy", "user"))
-        mock_self = MockSelf("public", self.local_user)
+        MockSelf = namedtuple("Self", ("privacy", "user", "recipients"))
         self.local_user.followers.add(self.remote_user)
         self.local_user.followers.add(another_remote_user)
 
+        mock_self = MockSelf("public", self.local_user, [])
         recipients = ActivitypubMixin.get_recipients(mock_self)
-        self.assertEqual(len(recipients), 1)
-        self.assertEqual(recipients[0], "http://example.com/inbox")
+        self.assertCountEqual(recipients, ["http://example.com/inbox"])
+
+        # should also work with recipient that is a follower
+        mock_self.recipients.append(another_remote_user)
+        recipients = ActivitypubMixin.get_recipients(mock_self)
+        self.assertCountEqual(recipients, ["http://example.com/inbox"])
 
     def test_get_recipients_software(self, *_):
         """should differentiate between bookwyrm and other remote users"""