Factor out DGU code from the dataset map

It now lives in the DGU extension:

https://github.com/datagovuk/ckanext-dgu/blob/master/ckanext/dgu/theme/public/scripts/dgu-dataset-map.js
Remove stuff from html.py

The rest will eventually go when we migrate the spatial search snippet.
[#6] Move to Leaflet for dataset map widget

For this particular use case Leaflet offered the best option. Also
solves the issue when showing extent covering the whole world.