This brings several improvements:
* The Dropbox script won't slow loading our app.
* If it fails to load, no traceback; Dropbox link just won't appear.
* For users with Dropbox disabled (most at this point), no loading at all.
(imported from commit e71ae5790fc85a185e622bdafb350109527b4eee)
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)