Adds a deprecation for $LAB usage in case plugins use it

Previously it was just removed which broke MathJax.

app/assets/javascripts/discourse/initializers/lab-deprecation.js.es6

@@ -0,0 +1,19 @@
+import loadScript from 'discourse/lib/load-script';
+
+export default {
+  name: 'lab-deprecation',
+
+  initialize() {
+    if (window.$LAB) { return; }
+
+    window.$LAB = {
+      script(path) {
+        Ember.warn('$LAB is not included with Discouse anymore. Use `loadScript` instead.');
+
+        const promise = loadScript(path);
+        promise.wait = promise.then;
+        return promise;
+      }
+    };
+  }
+};