add support for Leaflet.hash.js (automatic permalinks for current viewport)

kamba4requested to merge
robertharm:dev into master

Created by: robertharm

This PR includes the leaflet.js plugin https://github.com/mlevans/leaflet-hash - it automatically adds dynamic URL hashes to web pages with Leaflet maps for better shareability than the current permalink-link which users have to click manually to get a shareable URL.

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