Fix js-disabled submission in Chrome/Firefox

NOTE: I've tested in Firefox (works with/without javascript) but couldn't test in Chrome, due to browser bug whereby I can't reliably disable javascript -- please test if you can!


Fixes #932, which is caused by HTML5 validation firing for hidden
elements.

Sets #mapForm to novalidate by default, and reverts this in
fixmystreet.js.

(Note that the attributes 'validate' and 'novalidate', which are
honoured by the browsers' HTML5 validation) aren't to be confused with
the class (class="validate") which is presumably only picked up by
javascript, and therefore not problematic for the no-js case.