Replaces the old convoluted / confusing file structure with a neater
class based system. It should now be easier to understand the purpose
of each file.
Performance should see a small bump and it will be easier to extend
core features like the Customizer integration.
File names are more akin to WooCommerce core which improves consistency
/ recognisability.
Assets are all arranged into a single dir rather than being spread out
all over the place.
Deprecated `storefront_sanitize_hex_color()`.
Updated Gruntfile.js to reflect new structure.
