[#29] Add work unit edit link to invoices#new form.  Fix some always-pass specs in invoices#new view spec.
Redirect back after editing a work unit.  This implements #27.
[Finishes #29 and #27] Fix story spec broken by changed work unit edit flow.
Merge pull request #30 from TimePulse/29_edit_work_units_on_new_invoice

29 edit work units on new invoice