stream: Add do_change_can_remove_subscribers_group and field to objects.

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.

zerver/models.py

@@ -2552,6 +2552,7 @@ class Stream(models.Model):
         "name",
         "rendered_description",
         "stream_post_policy",
+        "can_remove_subscribers_group_id",
     ]
 
     @staticmethod
@@ -2561,6 +2562,7 @@ class Stream(models.Model):
 
     def to_dict(self) -> APIStreamDict:
         return APIStreamDict(
+            can_remove_subscribers_group_id=self.can_remove_subscribers_group_id,
             date_created=datetime_to_timestamp(self.date_created),
             description=self.description,
             first_message_id=self.first_message_id,
@@ -4364,6 +4366,7 @@ class AbstractRealmAuditLog(models.Model):
     STREAM_REACTIVATED = 604
     STREAM_MESSAGE_RETENTION_DAYS_CHANGED = 605
     STREAM_PROPERTY_CHANGED = 607
+    STREAM_CAN_REMOVE_SUBSCRIBERS_GROUP_CHANGED = 608
 
     # The following values are only for RemoteZulipServerAuditLog
     # Values should be exactly 10000 greater than the corresponding