Prevent duplicate comment submission - fixes #389
Prevent duplicate comment submission - fixes #389

Some people double-click on buttons in their browser, and this
would result in duplicate comments, since the comment was being
submitted with AJAX.  This commit disables submission on
submission, and reenables it on getting the response.

mysociety/fixmytransport,mysociety/fixmytransport,mysociety/fixmytransport,mysociety/fixmytransport,mysociety/fixmytransport,mysociety/fixmytransport