Upgrade to NodeJS 12

Update javascript deps to latest version.
Rebuild assets

- Keep `stylehint` in a lower version because starting at v9.0.0 it generates too much errors. We might fix them later.
- Don't touch `materialize-css` to avoid too much side effect. We might update it later (to 1.0.0).

.babelrc

@@ -1,5 +1,10 @@
 {
-    "presets": [
-        ["env", {"modules": false}]
+  "presets": [
+    [
+      "@babel/preset-env",
+      {
+        "modules": false
+      }
     ]
+  ]
 }