Revert "Revert "u3: clear cached mugs on cell edit""
Revert "Revert "u3: clear cached mugs on cell edit""

This reverts commit 5c211c682d9b9648b4d6113cbe97d066ad8fb032.

urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit