Don't depend on javascript for button to do a post. More semantic
Merge remote-tracking branch 'openaustralia_github/more_semantic_button' into develop
Remove FF button padding, use linear gradient for gradient on buttons in browsers that support it - with image backgrounds, we get tied up in issues about the line-height of input elements, which FF sets using !important. Fixes https://github.com/mysociety/whatdotheyknow-theme/issues/53