Mattermost integration blocks group destruction
On i2.online, if you create a group an enabled Mattermost integration with the group, and then try to delete the group, the group will be scheduled for deletion, but then never get deleted. In the admin panel for background_jobs you see:
NoMethodError: undefined method `delete' for #<Mattermost::Session:0x00007fb09a076f00>
Not sure when this first started, but I've reproduced in 10.2 and I swear this used to work in some 9.x.
/cc @bikebilly @zj @ayufan