MDL-40759 icons: convert uses of pix_url to pix_icon

Also change filepicker to use scaled svgs instead of sets of custom pngs. The svgs are taken from font-awesome.

question/engine/renderer.php

@@ -328,7 +328,7 @@ class core_question_renderer extends plugin_renderer_base {
             $alt = get_string('notflagged', 'question');
         }
         $attributes = array(
-            'src' => $this->pix_url($icon),
+            'src' => $this->image_url($icon),
             'alt' => $alt,
         );
         if ($id) {