Refine UI for ordering elements

[ehmatthes]

• Allow ordering of sas within an organization.
• Finish writing tests for modifying order.
  • Includes some refactoring.
• Don't display up arrow for first element, or down arrow for last element. Might need to go back to more logic in views, less in templates. Send eus in dictionary with cas as keys.
• Consider whether ca.get_parent() should always return sa, never sda. Is sda ever used as a parent? Would simplify some of the ordering code, including get_parent_order() and set_parent_order().
• Move get_parent_order() and set_parent_order() to utils.py.