Add Guardian Egyptian Web Medium to project

Soon to be used on the new fronts.

I've also included a map of the actual fonts that we use in webfonts.scss. Before we weren't passing anything to guss-webfonts(), which meant that before IE9 was loading **all** the fonts.
I grabbed this config from the readme in [guss-webfonts](https://github.com/guardian/guss-webfonts/) which looks like it was out of date. I've just updated to reflect the latest in `webfontjson.js`.

You can see what [`guss-webfonts`](https://github.com/guardian/guss-webfonts/blob/master/src/_webfonts.mixins.scss#L89) loads in by default in the [config](https://github.com/guardian/guss-webfonts/blob/master/src/_webfonts.config.scss#L125).