This was previously causing us to generate a traceback every time we
hit a duplicated zephyr due to CC'ing.
(imported from commit 240e1559655d0166dcd864e84649ab97b87a29ad)
Our API documentation says that we do, and it seems like it could be
useful to clients, so we might as well do it.
(imported from commit c391e4952a09d41df4dc06e3dc6ee094f774822b)
This needs to be deployed to both staging and prod at the same
off-peak time (and the schema migration run).
At the time it is deployed, we need to make a few changes directly in
the database:
(1) UPDATE django_content_type set app_label='zerver' where app_label='zephyr';
(2) UPDATE south_migrationhistory set app_name='zerver' where app_name='zephyr';
(imported from commit eb3fd719571740189514ef0b884738cb30df1320)