Add location picker for rooms, buildings and locations

[JessicaDierking]

As a user I want to be able to pick a location on the map when adding a room, building or location so that I don't have to find out the geocoordinates.

Acceptance:

• In the editors for rooms, buildings and locations there is a button for "select location on map"
• When clicking the button, the map opens and the user can place a marker
• When a marker is placed the editor opens again, with the marker's position entered

[JessicaDierking]