Fix off-canvas causing blank pages in iOS <= 6

Pulling in @dennisroethig's fix from here https://github.com/99designs/contests/pull/3555 so handover, projects and workbench can benefit (had reports of it in handover).

Left out the ```-webkit-overflow-scrolling: auto;``` from @dennisroethig's contests PR as it doesn't appear to relate to this bug, and it looks like @xzyfer wants to remove it from our normalize.

@xzyfer @andrewk