(WIP) Mentoring assessment mode

The work of assessment mode. 

Currently missing:

* Taking into account the max_attempts
* Make all light children have their own *weight* for score
* Test in client app 

CC @antoviaque if you want to review and give it a try in the LMS
Both `render_*template` functions render text, so this function call would deserve a more descriptive name - maybe `render_js_template`?