Users should only be allowed to have two active grading attempts

[CatarinaSilva]

Problem

When the portal is under pressure and grading takes longer, students tend to submit even more grading attempts. This creates a positive feedback that increases the load and ends up with a bunch of stuck gradings.

Proposal

We can fix that by limiting the grading attempts and returning an error to the students when they try to submit more than X attempts per unit.

Suggestion:

• Max 2 active grading per student per unit

[Gustavo-SF]

#255 and #256 for reference

[Gustavo-SF]

We can set the button as disabled when there are +2 grades unfinished.