Switch to SPA/XHR'd in model for non-SW experience

[addyosmani]

Atm, we revert to full static page reloads for browsers without Service Worker support. We can probably switch to a more SPA-like model without too much difficulty. We're otherwise opting for simplicity with the current approach with a focus on fast initial paint.

[gauntface]

Starting looking into this and it's not too much work, the one thing I'm struggling to find a nice solution for is what the router does when it's initialised.

At the moment it assumes the body area of the web app is empty and that when a page is loaded it'll call onStart() on the pageController for that route, which will grab the partial HTML and load it.

For first render we want static content to be served, so there is no need for onStart to be called or for the partial to be loaded.

The most obvious solution is to add a class to the main body element of 'static-content-loaded' which the router will simply skip on the start step and perhaps call onUpdate instead.

Thoughts on this approach? @addyosmani @paullewis