Migrate from Google Maps to Open Source alternative

[Benjamin-Mueggenburg]

Migrated for using google maps to Leafletjs (https://leafletjs.com/) for the interactive maps combined with Openstreetmap (https://www.openstreetmap.org) as the tile server. Have translated almost all of previous code for google maps api to the Leaflet api for handling markers and polylines. The google maps api has a marker label, which appears on the actual marker. This has not been yet implemented into leafletjs, so I was unable to translate that feature across.

Doing so fixes the errors: #39 #38 #19 Which were caused by the api key being deleted from the developer console. The openstreetmap tile server does not require an api key meaning the problem should not occur again.