use djangocms-maps (for Mapbox support) instead of djangocms-googlemaps

- we currently use djangocms-googlemaps which only supports google maps
- for google maps a credit card is required always, even though all of our (potential) clients are in the free tier and wont have to pay.
- therefore we prefer mapbox
- I propose to switch the djangocms-template project to use https://github.com/Organice/djangocms-maps