Add a method get_all_staff_in_all_packages() for more sane querysets

[profcturner]

A challenge is having a useful collection of staff in drop down menus.

• all staff can be overwhelming and increase the chance of incorrect staff being added
• package.get_all_staff() is used in many places, and works quite well but
• staff.get_all_staff_for_all_packages() is useful for admins working across several packages.

A new set of tests have been added to verify this behaviour.

I also found that if one created a non validating form in a formset, you couldn't delete it without an exception.