Add switch to only show exchanges with feedback on univ page

As we can see on the screenshot bellow, most of exchanges are 'empty': nossing was contributed by the student concerned by the exchange.

The objective here is to add a switch next to branch to show only exchanges with contributions. (the switch should be active by default and if no exchanges match add a button to easily switch back it off)

⚠ think about the behavior we want when coming back to the page, etc.