Nudge users to use aerial imagery

[danieldegroot2]

When creating a POI and/or improving the POI position accuracy

• add an explicit notice in the location step to tell users to check for available imagery.

On desktop, there should be enough width to add a text on the left/right side, along the lines of ('Do not copy information from copyrighted maps.') 'Check the layer menu (bottom left). Choose a proper aerial imagery layer (top left in menu).' 'Place the POI as close as possible to its actual location.' 'You can always improve the object's location later (in the object's detail pane) if needed.'

Or, especially for mobile, it could also be a short hint (i.e. '<= Tip: Add an aerial imagery layer').

If it is too repetitive, it could be limited to the first (few) additions.

• add some way to see the distance between objects. This is especially useful when there is insufficient aerial imagery. (Related https://github.com/streetcomplete/StreetComplete/issues/5765 )

These changes may further prevent POI in the middle of a road/car park if it is located inside a building or on the side of the road/car park. (adding blindly based on default basemap); example where I modified one https://overpass-api.de/achavi/?changeset=154774401