invoices: fixup n_spaces vs mc_stall_option

* extra/forms.py: do not clean-up the value of n_spaces
* extra/models.py: set default n_spaces to 0
* extra/views/mgmt.py: fix invoice details using the new M&C
  stalls amounts
* templaces/cams: do not show n_spaces if 0 and show
  mc_stall_option if not none + fix typo in
  invoice_hard_copy.html (dangling /div)