lp1686459 webstaff: item editor form validation

lp1686459 webstaff: item editor form validation

Specifically for stat cats, we mark the select statements as required if the
corresponding stat cat is required, but we can also start using HTML5 validation
tests elsewhere in the form now as well.

I also fixed the <NONE> entries so that they display instead of blank entries,
and forced the Price, Cost, and Deposit Amount fields to be of input type number,
though you could still enter invalid numbers here from the database's POV, so
better validation tests are still needed there.

Signed-off-by: Jason Etheridge <jason@equinoxinitiative.org>