Add grading capabilities to mentoring blocks

This introduces grading for the mentoring blocks.

The block grade is the average of its children grades. They are as follows:

* Free text answer: answered = 1, unanswered/empty = 0
* MCQ: correct choice selected = 1, otherwise = 0
* MRQ: (number of correctly selected or unselected choices) / (total number of choices available)

The weight property is the maximum block grade and is set as the `max_grade` xml attribute of the `mentoring` object.

    <mentoring url_name="mentoring-0a06b184-9801-4f93-8c88-dedd72edadb0" max_grade="20">

If not set it defaults to 0 making the block effectively ungraded.

~~**Issue 1**: At the moment it looks like it's not possible to edit Settings (the mode is disabled) even if there is a `Scope.settings` Field. I'm probably missing a option to turn that on (pinging @aboudreault for help, maybe?)~~

**Issue 2**: Should grading be enabled if weight is defined like this? Or with a Block Setting? Or a global one? Or always? @antoviaque