We were previously using the true count of unread message for HTML
emails but the number of conversations for plaintext. Make them
consistent.
(imported from commit efb140abcc95faf00631c03580f518ca4d8ef58c)
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)