MDL-42404 behat: Adding edit rubrics feature

Automates the following MDLQA-1185, MDLQA-1188
and MDLQA-1427. All of them related with the
expected outcomes after editing rubrics definitions
that were already used to grade students.
MDL-42404 behat: Adding rubrics step definitions
MDL-42404 behat: Adding feature to test reusing user's rubrics

Automates MDLQA-1187, teachers can reuse their
rubrics in other activity.
MDL-42404 behat: Covering MDL-39710 with behat features

I've contributed to an extremely large an ugly scenario
in favour of performance as separating it in another
scenario or feature would add 3 or 4 minutes to the suite.