MDL-40759 icons: Rename activity_icon

image_icon is a better name because there are some valid uses for these other than for activity icons.

E.g. Old JS code that is not worth re-writing.

blocks/feedback/block_feedback.php

@@ -53,7 +53,7 @@ class block_feedback extends block_list {
             $courseid = SITEID;
         }
 
-        $icon = $OUTPUT->activity_icon('icon', get_string('pluginname', 'mod_feedback'), 'mod_feedback');
+        $icon = $OUTPUT->image_icon('icon', get_string('pluginname', 'mod_feedback'), 'mod_feedback');
 
         if (empty($this->instance->pageid)) {
             $this->instance->pageid = SITEID;