Add events when scales and letters are managed

Automated testing (CiBoT will detect any problem, no manual action is needed): 1. Run unit tests: vendor/bin/phpunit core_grade_events_test grade/tests/events_test.php 2. Run the behat tests: grade/tests/behat/grade_letter_logging.feature and grade/tests/behat/grade_scales_logging.feature Manual testing: 1. Open "Live logs" or "Logs" report 2. In a course create and update a scale. Make sure there are no notices or errors, and each action is reflected in the logs report. Make sure the links from the logs report go to the editing page for the affected scale and he event context is the course. 3. Delete a scale in the course. Make sure the event appeared in the log report and there is no link to the scale. Make sure the event context is the course. 4. In a course create and update grade letters. Make sure there are no notices or errors, and each action is reflected in the logs report. Make sure the links from the logs report go to the editing page for the affected letter and the event context is the course. 5. Delete a grade letter in the course. Make sure the event appeared in the log report and there is no link to the grade letter. Make sure the event context is the course. 6. Repeat steps 1-5 on the site level. Verify that event context is "system" for all six types of events.