Add overview all sensors

[elma13]

It would be great if it is clear or the user that all the data for the netherlands can be analysed, in parts. So the preloaded data is just an example, there is more to choose from.

• Add more information to the map in the start/home page: so that it is clear that whole Netherlands can be used.
• Either: all markers of all sensors, highight markers in gemeente -> can we have the locations in the database (can be updated automatically weekly?)
• Or: use the gemeente-grenzen and highlight the choosen gemeente -> how to do with a selected project???
• Or: another solution to show the people that we have an example dataset loaded, but that there is more available