Create extension to render JIRA TAG links

See https://github.com/showdownjs/showdown/wiki/Extensions

app/static/js/items.js

@@ -6,7 +6,17 @@ $(document).ready(function() {
     scrollBy(0, -100);
   }
 
+  // Showdown extension to render links to JIRA TAG project
+  var jira_tag = {
+      type: 'lang',
+      regex: /(TAG-\d+)/g,
+      replace: '[$1](https://jira.esss.lu.se/browse/$1)'
+  };
+  // Register the extension
+  showdown.extension('jira_tag', jira_tag);
+
   var converter = new showdown.Converter({
+    extensions: ['jira_tag'],
     simplifiedAutoLink: true
   });