Move generic util into lib

Might come in handy for the admin section later on

js/forum/src/utils/extractText.js

@@ -1,19 +0,0 @@
-/**
- * Extract the text nodes from a virtual element.
- *
- * @param {VirtualElement} vdom
- * @return {String}
- */
-export default function extractText(vdom) {
-  let text = '';
-
-  if (vdom instanceof Array) {
-    text += vdom.map(element => extractText(element)).join('');
-  } else if (typeof vdom === 'object') {
-    text += extractText(vdom.children);
-  } else {
-    text += vdom;
-  }
-
-  return text;
-}