Improve visual appearance of the elevation popup

[otbutz]

Old:

New:

• properly align
• add images
• separator for additional actions

[easbar]

The elevation popup is also ugly:

Before it looked like this:

It used to be an <svg> layer on top of the map box, but I was not sure how to do this with Mapbox, hence we just use the default popup currently.

[easbar]

After the OpenLayers migration the popups look like this now:

We still need to improve this. See comments here, here and here

[otbutz]

I think we can close this one. The current state looks pretty good.

[easbar]

I kind of liked this old elevation 'flag', but yeah there are probably more important things to work on :)

[easbar]

I'd rather keep it open still, there are also some related todos in the code. But I'll change the title and remove the context menu part.