invalidate user's cached groups on membership update

fixes CNVS-5256

test plan
- as a teacher, add a user to a group
- wait 15 minutes or clear the "Courses & Groups" cache entry
- as the user, ensure that the new group shows up in the "Courses
 & Groups" menu
- as the teacher, remove the user from the group
- wait 15 minutes or clear the "Courses & Groups" cache entry
- as the user, ensure that the new group does not show up in the
 "Courses & Groups" menu

Change-Id: I091775f50783e0f9bb99a4adbc3e726a7f78be89
Reviewed-on: https://gerrit.instructure.com/45603
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>

app/models/group_membership.rb

@@ -40,8 +40,10 @@ class GroupMembership < ActiveRecord::Base
   after_save :touch_groups
   after_save :update_cached_due_dates
   after_save :update_group_leadership
+  after_save :invalidate_user_membership_cache
   after_destroy :touch_groups
   after_destroy :update_group_leadership
+  after_destroy :invalidate_user_membership_cache
 
   has_a_broadcast_policy
 
@@ -185,6 +187,10 @@ class GroupMembership < ActiveRecord::Base
      enrollments.any?{ |e| e.user == self.user && e.course == self.group.context })
   end
 
+  def invalidate_user_membership_cache
+    Rails.cache.delete(self.user.group_membership_key)
+  end
+
   alias_method :destroy!, :destroy
   def destroy
     self.workflow_state = 'deleted'

app/models/user.rb

@@ -1717,9 +1717,13 @@ class User < ActiveRecord::Base
     end
   end
 
+  def group_membership_key
+    [self, 'current_group_memberships', ApplicationController.region].cache_key
+  end
+
   def cached_current_group_memberships
     self.shard.activate do
-      @cached_current_group_memberships = Rails.cache.fetch([self, 'current_group_memberships', ApplicationController.region].cache_key) do
+      @cached_current_group_memberships = Rails.cache.fetch(group_membership_key) do
         self.current_group_memberships.shard(self.in_region_associated_shards).to_a
       end
     end