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.

**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 always enabled like this? Or with a Block Setting? Or a global one? Or if weight is defined? @antoviaque