allow media comments for discussion group assignments

fixes #5550

Change-Id: I80a4714c7f690b365bded8e38f62247f0bda696f
Reviewed-on: https://gerrit.instructure.com/5487
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Brian Whitmer <brian@instructure.com>

app/models/group.rb

@@ -368,4 +368,8 @@ class Group < ActiveRecord::Base
     context.imported_migration_items << item
     item
   end
+
+  def allow_media_comments?
+    true
+  end
 end