Make rating from leaderboard persisted columns + add index (fixes #200)

Author: Brutus5000

migrations/V72__rating_as_generated_column.sql

@@ -0,0 +1,4 @@
+ALTER TABLE `ladder1v1_rating` ADD COLUMN `rating` FLOAT GENERATED ALWAYS AS (mean - 3 * deviation) STORED;
+ALTER TABLE `ladder1v1_rating` ADD INDEX `rating` (`rating`);
+ALTER TABLE `global_rating` ADD COLUMN `rating` FLOAT GENERATED ALWAYS AS (mean - 3 * deviation) STORED;
+ALTER TABLE `global_rating` ADD INDEX `rating` (`rating`);