Show warning message when changing target branch &

Currently, when the user changes the target branch, it will display an alert

An improvement would be to only display the warning if the user has add some new rules (in other words when the approval rule form is dirty).

Initial Form	Clean Form	Dirty
-	Warning will be displayed if target branch is changed	No warning is displayed if target branch is changed

For reference, the first part of this iteration is in this MR > !27738 (closed)