MDL-47146 grades: fix navigation bar in grades setup pages

Part of: MDL-46576
MDL-47146 grades: change action buttons on setup screen

Part of: MDL-46576
MDL-47146 grades: change columns on setup page

Part of: MDL-46576
MDL-47146 grades: Removed unused setup columns
MDL-47136 grades: correcly align the select column
MDL-47146 core_grades: Use a checkbox to override weights

Part of: MDL-46798
MDL-47146 core_grades: Improve labels of the fields on setup screen
MDL-47146 core_grades: Behat test for Natural weights on setup screen
MDL-47146 grades: improve navigation on grade pages