fix stall fields in add/edit invoice templates

When defined, show n_spaces, n_tables, mc_stall_option and
plot_type in add_invoice.html and edit_invoice.html
mgmt.py: fix stall invoice amount + tidy-up

Fix stall invoice amount in AddInvoiceView for both
stall types (M&C and FF) using hard-coded dictionaries.

Also fix code formatting and tidy-up details put in
invoice html page.
rename invoice hard_copy -> html

* mgmt_urls.py: rename hard_copy -> html
* mgmt.py: rename InvoiceHardCopyView -> InvoiceHTMLView
* templates/cams: rename with html instead of hard_copy
extra/views/mgmt.py: fix invoice details again