Update leaflet.fullscreen to 3.0.0 (#1818)

Fixes #1687.

Note: the CDN was changed from cdnjs to jsdelivr because an issue
with the former means that the fullscreen icon is broken for all
versions >= 2.2.0 and <= 3.0.0.

folium/plugins/fullscreen.py

@@ -40,13 +40,13 @@ class Fullscreen(JSCSSMixin, MacroElement):
     default_js = [
         (
             "Control.Fullscreen.js",
-            "https://cdnjs.cloudflare.com/ajax/libs/leaflet.fullscreen/1.4.2/Control.FullScreen.min.js",
+            "https://cdn.jsdelivr.net/npm/leaflet.fullscreen@3.0.0/Control.FullScreen.min.js",
         )
     ]
     default_css = [
         (
             "Control.FullScreen.css",
-            "https://cdnjs.cloudflare.com/ajax/libs/leaflet.fullscreen/1.4.2/Control.FullScreen.min.css",
+            "https://cdn.jsdelivr.net/npm/leaflet.fullscreen@3.0.0/Control.FullScreen.css",
         )
     ]