Allow user cancellation of PI request

[bryank-cs]

It frequently happens that someone requests a PI account when they intended to join a group. Currently an admin must deny the request in order to allow the user to try again. It would be nice if the user could request to join a group without intervention, and the easiest implementation may be to allow them to cancel the PI request. Adding the ability to join a different group even with a pending PI request is also ok.