Makes the editor focusable.

It's related to CachetHQ/Cachet#2749
On the incident template creation the editor was not focusable, now it
is.
The problem was the initialization of the editor that was done before
the DOM is fully loaded.
Using addEventListener and DOMContentListener fixes the problem and is
compatible from IE9.

resources/views/dashboard/templates/add.blade.php

@@ -9,13 +9,14 @@
 <script src="https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.8.0/mode/twig/twig.min.js"></script>
 
 <script>
-(function() {
-    var editor = CodeMirror.fromTextArea(document.getElementById('cm-editor'), {
-        lineNumbers: true,
-        mode: 'twig',
-        lineWrapping: true
-    });
-}());
+//Initializes the editor only once the DOM is loaded.
+window.addEventListener("DOMContentLoaded", function(e) {
+     var editor = CodeMirror.fromTextArea(document.getElementById('cm-editor'), {
+         lineNumbers: true,
+         mode: 'twig',
+         lineWrapping: true
+     });
+});
 </script>
 @stop