Set box size for CSS spinner to viewport size.

Loading indicator, spreads over whole screen estate to intercept mouse
events until view is rendered.

Fixes the following problems, by preventing interaction during page load with:

- The language selector. Unfortunately, the language list is not loaded yet.
- The Login DropDown. This is not displayed properly until the page is fully loaded
- The Cart. There's no point in interacting with the cart during application initialization.