Create form.html

Form elements to add to sign up box just below the petition text. This form will post to the Sendy instance for this campaign.

Please check my code carefully, not sure it's totally right, particularly the name and id input attributes.

Jonathan's original instructions are here: https://docs.google.com/a/okfn.org/document/d/1MO9HjWAt6sEjdt6tSAoL8iRIah9VSVo_LqGSzJRBfQ0/edit#heading=h.p5u3gwiqbtgm