Add support for source maps with less.php

Switch to a theme using lessphp (e.g. theme_more) Enable theme designer mode In a browser supporting sourcemaps, open the developer tools (Currently: Chrome, and Firefox 32/nightly); Enable the sourceMap support: Chrome: https://developers.google.com/chrome-developer-tools/docs/css-preprocessors#toc-enabling-css-source-maps FirefoxNightly: https://hacks.mozilla.org/2014/02/live-editing-sass-and-less-in-the-firefox-developer-tools/ Refresh the page Inspect some styles and click through to view the sourcemap (generally by clicking on the foo.less file) Confirm that the sourcemap looks to be for the correct thing Repeat for a range of: moodle-specific item styles; and bootstrap styles. Bask in the gloriousness that it sourceMaps. Switch to a different theme (one which does not use lessphp) Browser around some more Confirm that the sourceMaps are no longer present Switch back to theme_more and confirm that sourceMaps are once again available Disable theme designer mode Browser around some more Confirm that the sourceMaps are no longer present