Create a fast-loading map with the locations of all LaPrimaire supporters

[thibauld]

We had a map here: https://laprimaire.org/carte/
Unfortunately, we now have 11.000+ markers to display on the map and when I tried to do it via the traditional google map editor, the end result takes 33s to load..... totally unusable. So, for now, the map only displays ~1/8 of the cities where supporters are.

The idea is to implement one of these techniques to create a fast-loading yet complete map of LaPrimaire supporters: https://developers.google.com/maps/articles/toomanymarkers