Add Respond.js to support IE8 and other older browsers which do not understand CSS @media queries, as suggested by seaside98 on Brickimedia/brickimedia#224.

Also fixed the Flash of Unstyled Content, which was caused by the CSS and JS being combined into the same module and loaded via addModules() instead of the CSS being loaded via addModuleStyles() in setupSkinUserCss(). Moved JS loading into initPage(), which is somewhat copy-pasted from the core Vector skin.

Removed one unnecessary global in the BeforePageDisplay hook on the main setup file.