Originally this cache was used to transmit data from Django to Tornado
(and also for general message caching purposes), but now nothing
actually reads from this cache, so we can eliminate it.
Apparently, the message cache we were filling was completely useless
and unused, and furthermore, the cache we were filling as part of
restarting the server was also totally useless, since it didn't have
the messages users would be requesting.
This changes the type annotations for the cache keys in Zulip to be
consistently text_type, and updates the annotations for values that
are used as cache keys across the codebase.
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)