MDL-47110 core_grades: Normalisation of grade weights.

This includes behat tests for this change.

Part of: MDL-46576
MDL-47110 grades: more behat tests
MDL-47110 core_grades: Update to auto_adjust_weights

Part of: MDL-46576
MDL-47110 Grades: Change how updated weights notifications are detected.

Part of: MDL-46576
MDL-47110 Grades: Get the autoupdateweight behat tests to pass.

Part of: MDL-46576