R1:cd677161e16a

Do not CC users without permissions to view an object

Summary:
Ref T4411
I'm not quite sure if this is the right place for this as it will be difficult to provide proper user feedback of why we removed a particular subscriber.
Is the ApplicationTransactionEditor generally the right place to extract mentioned phids in comments?
On the other hand in some cases we cannot really give user feedback why a user was not subscribed (e.g.: commits & diffs)

Adding a diff to a repo where the user mentioned has no…