Fixed: fullscreen layout issues on mobile devices

Now, zoom controls in the toolbar display as expected on mobile devices with small screens.

* In fullscreen, div.diva-tools-left now displays before div.diva-tools-right, so that layout doesn't break on small screens.

* Media queries were added to prevent the slider from pushing div.diva-tools-right off of the toolbar on small screens (less than 480px, which covers small phone-like devices.)