if state is unknown on load, then work out which action buttons to show
if state is unknown on load, then work out which action buttons to show

previously, it'd always show nothing-selected-buttons - however, if
some items were checked (due to being selected previously, and loading
with a form error message), it would be showing the wrong buttons.

Now, if the state is unknown, work out which state to show by counting
checkboxes, the same as when someone presses the cancel button.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin