Add "send goodbye message?" control to mass removal page

Just as with mass subscribe, the admin may want to perform a removal differently from the prescription of the normal removal from the list. Specifically, the admin may prefer to send or not send a goodbye message where the list specifies the opposite.

Performing this task requires updating both the page template for mass removal and the view. The corresponding code for mass subscribe is a useful model.

Mass subscription has multiple such controls. The goodbye message is the only control useful for mass removal I can think of offhand, but please spend a minute or two thinking about whether there are others after looking at the code.