Add Javascript feature detection for oninput

This commit adds a feature detection for the `oninput` event, which isn’t
supported in older browsers[1].

This means that the code that highlights placeholders will only be run in
browsers that support the `oninput` event.

1. http://caniuse.com/#feat=input-event

app/assets/javascripts/highlightTags.js

@@ -1,6 +1,10 @@
 (function(Modules) {
   "use strict";
 
+  if (
+    !('oninput' in document.createElement('input'))
+  ) return;
+
   const tagPattern = /\(\([^\)\(]+\)\)/g;
 
   Modules.HighlightTags = function() {