Generate a default map view with Leaflet/Turf

[amcewen]

Given Jekyll will be serving up static pages, we need to know what those pages look like. As a first step, creating an HTML page that loads Leaflet.js (and Turf.js? I'm not sure what each of those provide, and so whether we need both?) and shows an OpenStreetMap map of Liverpool.

Then load an isochrone and show that on the map too. This comment has a URL to fetch a sample isochrone

[amcewen]

Turf.js seems to do more analysis of map data, and Leaflet.js the main showing tiles, drawing geoJSON areas, etc. So I think we can start with Leaflet and add Turf if we need some of the more advanced features it has.

I've got the basics done, so next is to pull that into Jekyll in #2.