Disable contact form submit after click

See mysociety/alaveteli@ce8cb9ea92d43002262099e765cccd064d7ea522

Users sometimes repeatedly click submit if the form takes a while to
submit. This disables the submit button with a message so that users
know the form is submitting.
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 remote-tracking branch 'origin/duplicate-contact-form-submissions' into develop
Merge branch 'contact-form-honeypot' into develop
Merge branch 'develop'