Detect enter key press for geocoding preview.

Test users intuitively press enter after filling the address form, expecting a geocoder preview.
Use custom model event instead of callback to decouple geocoding.
Split clearing of preview map, geocoding and preview tile display.

- With this separation, the tile can be displayed without geocoding if the location is already known.
Remove automatic geocoding of places and make latitude/longitude regular, accessible attributes.
Merge branch 'feature/optimize-geocoding-165' into prototype