MDL-69087 core_h5p: add renderer to let personalize styles

Existing mod_hvp pluging has a renderer to let Moodle instances
alter styles, Javascript, semantics (fields in the editor) and
content: https://github.com/h5p/moodle-mod_hvp/blob/stable/renderer.php

The approach for core_h5p is exactly the same, to let people
to reuse their existing code.

h5p/ajax.php

@@ -44,6 +44,9 @@ $action = required_param('action', PARAM_ALPHA);
 $factory = new factory();
 $editor = $factory->get_editor();
 
+// Set context to default system context.
+$PAGE->set_context(null);
+
 switch ($action) {
     // Load list of libraries or details for library.
     case 'libraries':