[C2] Add min scores to gradebook elements

[ywarnier]

The current (1.11.x) way of configuring the attribution of certificates/skills to users is based on a "weighted average" of scores in the different elements configured in a gradebook (gradebook_evaluation.weight and gradebook_link.weight).

However, as reported many times by our great community, this does not satisfy a large number of cases where we would like to establish minimum scores for some of the elements and just prevent the attribution of certificates/skills if this particluar minimum score has not been achieved for the element that has it.

• Add a min_score (float) field to the gradebook_evaluation and gradebook_link tables
• Update the certificate/skill attribution code to verify that all elements with a min_score are effectively achieved with at minimum that min_score. Otherwise, do not generate the certificate/skill
• In the results table, mark in red those elements that have not been completed with that minimum score