Frontend user page URL should include username rather an ID

[alyssadelaine]

So instead of /users/123456, the URL should be /users/TheirUserName

[kamicut]

This one might be tricky, OSM usernames can change (and OSM ids don't), so we have to make sure the latest username in the OSMesa data is associated to the proper OSM id. Similar issue in missingmaps https://github.com/MissingMaps/missingmaps.github.io/issues/265