zilencer: Tie RemotePushDeviceToken to RemoteRealm at registration.

This consists of the following pieces:
1. Makes servers using the bouncer send realm_uuid in requests for token
   registration. (Sidenote: realm_uuid is already sent in the "send
   notification" codepath as of
   48db4bf854)
2. This allows the bouncer to tie RemotePushDeviceToken to the
   RemoteRealm with matching realm_uuid at registration time.
3. Introduce handling of some potential weird edge cases around the
   realm_uuid and RemoteRealm objects in get_remote_realm_helper.

zilencer/models.py

@@ -80,6 +80,8 @@ class RemotePushDeviceToken(AbstractPushDeviceToken):
     user_id = models.BigIntegerField(null=True)
     user_uuid = models.UUIDField(null=True)
 
+    remote_realm = models.ForeignKey("RemoteRealm", on_delete=models.SET_NULL, null=True)
+
     class Meta:
         unique_together = [
             # These indexes rely on the property that in Postgres,