Add unread_msgs to the initial state data.

We are adding a new list of unread message ids grouped by
conversation to the queue registration result. This will allow
clients to show accurate unread badges without needing to load an
unbound number of historic messages.

Jason started this commit, and then Steve Howell finished it.

We only identify conversations using stream_id/user_id info;
we may need a subsequent version that includes things like
stream names and user emails/names for API clients that don't
have data structures to map ids -> attributes.

zerver/models.py

@@ -1272,6 +1272,14 @@ class AbstractUserMessage(ModelReprMixin, models.Model):
         abstract = True
         unique_together = ("user_profile", "message")
 
+    @staticmethod
+    def where_unread():
+        # type: () -> str
+        # Use this for Django ORM queries where we are getting lots
+        # of rows.  This customer SQL plays nice with our partial indexes.
+        # Grep the code for example usage.
+        return 'flags & 1 = 0'
+
     def flags_list(self):
         # type: () -> List[str]
         return [flag for flag in self.flags.keys() if getattr(self.flags, flag).is_set]