The e-mail forwarder will use this. Set it to nullable temporarily to
accomodate existing streams; later commits will a) provide a script to
give all streams a token, and b) make the field non-null.
Realm administrators will eventually have a UI to regenerate stream
tokens.
(imported from commit a084d0a7012eb9665e4da095cbc46aa9ef354eaa)
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)