Condense Database Migrations (#17103)

* Condense Database Migrations

Condenses the Database Migrations into a new initial schema.
Also adds some instructions to the Database Readme for existing setups.

* Update readme/delete changed migrations since pr creation

* Reworks a bit how the previous migrations are handled and updates the instructions accordingly

* Expand the readme with baseline/migrate

---------

Co-authored-by: Werner <Arrow768@users.noreply.github.com>

SQL/migrate-2017/V061__custom_items_database.sql

@@ -0,0 +1,19 @@
+--
+-- Implemented in PR TODO.
+-- Adds a `ss13_characters_custom_items` table to replace the old custom item system.
+--
+
+CREATE TABLE `ss13_characters_custom_items` (
+	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
+	`char_id` INT(11) NOT NULL,
+	`item_path` VARCHAR(255) NOT NULL COLLATE 'utf8mb4_unicode_ci',
+	`item_data` LONGTEXT NOT NULL COLLATE 'utf8mb4_bin',
+	`req_titles` LONGTEXT NULL DEFAULT NULL COLLATE 'utf8mb4_bin',
+	`additional_data` VARCHAR(255) NULL DEFAULT NULL COLLATE 'utf8mb4_unicode_ci',
+	PRIMARY KEY (`id`) USING BTREE,
+	INDEX `FK_ss13_characters_custom_items_ss13_characters` (`char_id`) USING BTREE,
+	CONSTRAINT `FK_ss13_characters_custom_items_ss13_characters` FOREIGN KEY (`char_id`) REFERENCES `ss13_characters` (`id`) ON UPDATE CASCADE ON DELETE CASCADE
+)
+COLLATE='utf8mb4_unicode_ci'
+ENGINE=InnoDB
+;