Let user know before clicking Add that you are about to steal an issue

In #4001 (closed) we're allowing users to undo adding an issue to an epic if that issue already belonged to another epic. But they only know that after pressing the “Add” button.

A better experience would be to check as soon as the user pastes the issue link and show a warning message saying This issue will be removed from epic [&1337] and added to this one. This is a faster interaction model with instant validation and no confirmations (i.e. less friction).