Add honeypot spam protection to contact form

Requires mysociety/alaveteli@6fbaed8b841d081681fe970abab38b648fed5253

Intercepts the request and redirects to the homepage if the comment
field is filled in on the contact form.
Merge branch 'duplicate-contact-form-submissions' into contact-form-honeypot
Merge branch 'contact-form-honeypot' into develop