Todolist panel in admin editor

This PR adds a TODO panel in the admin editor, by clicking on each TODO the involved file will be opened (or selected if already open) and the cursor will be placed at the right line.
Further commits will come to:
- improve style
- improve compatibility (tested only Chrome/Firefox on ubuntu)
- support other keyword (ex: FIXME and so on)
- support other file types (at the moment works for .py only)