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
MDL-47430 core_grades: Handle negative grademin with natural weighting

Part of: MDL-46576
MDL-47430 core_grades: Adding behat test for alternative mingrade

Part of: MDL-46576
MDL-47146 core_grades: fix class names (contd)

Part of: MDL-46576
MDL-47492 behat: refer to table column indexes instead of names
MDL-47492 core_grades: Behat test for using scales and mingrade in gradebook
MDL-47430 core_grades: rename strings in Behat tests