Known issues:
* No support for whitelabeling in the email
* No whitelabeling for any externally-visible branding
(imported from commit 9eab7b0744e56a87007b8621a8bb18bbb1080256)
Also compactify and reorder it a bit to ensure that emoji and
@-mentions are near the top, since they are compact and important to
all users.
(imported from commit 7a3e6c915d1ef1461a43302c71bda74852566ffc)
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)