Remove SplitView preview option if the device cannot use it.

Only show the Split View option in the "Change view mode" menu if the device has a screen width capable of using it. The width is only checked when the CMS initially loads; manually resizing the window afterwards will not disable the option.

This builds on https://github.com/silverstripe/silverstripe-framework/pull/3291 but adds a js library [respond.js](https://github.com/scottjehl/Respond) to all LeftAndMain admins that is a "polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)"
Should probably just be included directly rather than using a submodule (especially since there's no .gitmodules)