Allow forms to disable autofill in all browsers

New variable added to form to specify whether it should autofill or
not. This updates the modal forms autocomplete attribute. Hidden
fields are required because Chromium v34+ based browsers ignore the
autocomplete=off form attribute, so the hidden fields catch autofill
data. Any form without self.no_autocomplete = True will just act
like before.

Change-Id: Ibb1722023eea8f57312e1133939d1f75cd909467
Closes-Bug: 1352459
Merge "Allow forms to disable autofill in all browsers"