Warn the editor when the mentioned actor cannot view the page

If a mentioned actor cannot view the page/entity in which he is mentioned, no notification is sent (to avoid leaking the name and url of the page as well as some samples of the page content).

From the editor point of view, that can be missleading because he expect the mentioned actor to receive and possibly react to the the mention.

Consequently, when a mentioned actor cannot view the page/entity in which he is mentioned, a visual clue must be displayed on the mention to warn the editor that no notification will be sent for the mention.