When you read messages in a narrow and then un-narrow, collapse
adjacent messages read in the narrow into a summary row that can
be clicked to expand those messages.
Scoped to staging with feature flags.
The implementation of this within our current MessageList is rather ugly.
(imported from commit bcb3a39d8c0c334136fe86318f18ead03f0f50bf)
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)