Anti spam honeypot

Fixes theodi/shared#68 and theodi/open-orgn-services#278

This should hopefully rectify all the issues we've been having with spam in Capsule. It's a hidden checkbox, which spambots should automatically check. If the field is present and equal to anything but "0" (the default 'off' value for checkboxes), the form errors. I've used this method before and it's been pretty successful in the past.

If we're still having problems after a few weeks, then we can try something more heavyweight like a captcha or Akismet, but this is a nice lightweight solution for now.