Move database migrations to SQLx

migrations/20220730000000_expand_join_sounds.sql

@@ -0,0 +1,10 @@
+CREATE TABLE join_sounds (
+    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
+    `user` BIGINT UNSIGNED NOT NULL,
+    `join_sound_id` INT UNSIGNED NOT NULL,
+    `guild` BIGINT UNSIGNED,
+    FOREIGN KEY (`join_sound_id`) REFERENCES sounds(id) ON DELETE CASCADE,
+    PRIMARY KEY (`id`)
+);
+
+INSERT INTO join_sounds (`user`, `join_sound_id`) SELECT `user`, `join_sound_id` FROM `users` WHERE `join_sound_id` is not null;