Let user know before clicking Add that you are about to steal an issue
In #4001 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).