feat(widget): update Mapbox API due to classic style deprecation

Since June 1, 2020, the Mapbox classic styles are deprecated and may be removed in the future.
This change updates the Leaflet code to use the currently supported Mapbox style API according to the official documentation at https://docs.mapbox.com/help/troubleshooting/migrate-legacy-static-tiles-api/

See #1854

wowchemy/assets/js/academic.js

@@ -266,10 +266,12 @@
       } else {
         let map = new L.map('map').setView([lat, lng], zoom);
         if (map_provider == 3 && api_key.length) {
-          L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
+          L.tileLayer('https://api.mapbox.com/styles/v1/{id}/tiles/{z}/{x}/{y}?access_token={accessToken}', {
             attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
+            tileSize: 512,
             maxZoom: 18,
-            id: 'mapbox.streets',
+            zoomOffset: -1,
+            id: 'mapbox/streets-v11',
             accessToken: api_key
           }).addTo(map);
         } else {