This commit adds do_change_can_remove_subscriber_group function for
changing can_remove_subscribers_group field of a stream. We also add
can_remove_subscribers_group_id field to stream and subscription
objects.
This function will be helpful for writing tests in next commit.
We would add API and UI support to change this setting in further
commits.
This commit removes "role" field from subscription
objects since we are not moving forward with stream
administrator concept and instead working on new
permssions model as per #19525.
Sahil Batra
Zixuan James Li
Anders Kaseorg
Steve Howell