[IMPLEMENTED] Hide already selected items from content selector


#1

See: [IMPLEMENTED] Unique selection on Relation Field


#2

@mayvelous I have implemented it. I decided to mark the selected items with a checkbox, instead of just hiding it, because …

a) It is easier to implement
b) You don’t have to ask yourself “where the hell is XYZ”

It is not deployed to cloud yet, but pushed to master.