Introduce statistics also on OSM contributors

[MarcoMinghini]

An important variable to judge the (intrinsic) quality of any OSM object is the experience of the user(s) who have made edits on that objects.

This can be included in the web application in at least two ways:

• in the popup, by showing (next to each user who has edited the object) the number of edits made by that user. This would provide useful, but only qualitative information.
• in the map (perhaps together with other aggregated statistics, see Visualize aggregated quality results on the bbox #5), some aggregated measures about the users who have made edits to the OSM nodes and ways in the selected bbox: for instance the total number of users; the mean/median/standard deviation of the number of users who made edits to the OSM nodes and ways in the bbox; and their average number of OSM edits (in total). This would also provide useful, quantitative information.