Add dynamic polyfills for older browsers (#2985)

Fixes #2941

app/javascript/mastodon/polyfills.js

@@ -0,0 +1,18 @@
+import 'intl';
+import 'intl/locale-data/jsonp/en.js';
+import 'es6-symbol/implement';
+import includes from 'array-includes';
+import assign from 'object-assign';
+import isNaN from 'is-nan';
+
+if (!Array.prototype.includes) {
+  includes.shim();
+}
+
+if (!Object.assign) {
+  Object.assign = assign;
+}
+
+if (!Number.isNaN) {
+  Number.isNaN = isNaN;
+}