Use plugins toolkit whenver possible on plugin.py
[#4] Register resources only once for all plugins

spatial_metadata will load the resources (public, templates and
resources) for all plugins to use, as it needs to be loaded anyway.
[#6] Update dataset map to be a pure snippet

No need to load an extension.
Update README

WMS preview, dataset map snippets, formatting fixes.
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.
Merge branch 'release-v2.0' into 2.0-validation-changes