Add support for system mail deliveries

[pacampbell]

Added support for system mail to deliver items to the player.

Database Migration

CREATE TABLE IF NOT EXISTS "ddon_system_mail" (
    "message_id"    INTEGER NOT NULL,
    "character_id"  INTEGER NOT NULL,
    "message_state" INTEGER NOT NULL,
    "sender_name"   VARCHAR(256) NOT NULL DEFAULT "",
    "message_title" VARCHAR(256) NOT NULL DEFAULT "",
    "message_body"  VARCHAR(2048) NOT NULL DEFAULT "",
    "send_date" INTEGER NOT NULL DEFAULT 0,
    FOREIGN KEY("character_id") REFERENCES "ddon_character"("character_id") ON DELETE CASCADE,
    PRIMARY KEY("message_id" AUTOINCREMENT)
);

CREATE TABLE IF NOT EXISTS "ddon_system_mail_attachment" (
    "message_id"    INTEGER NOT NULL,
    "attachment_id" INTEGER NOT NULL,
    "attachment_type"   INTEGER NOT NULL,
    "is_received"   INTEGER NOT NULL DEFAULT 0,
    "param0"    VARCHAR(256) NOT NULL DEFAULT "",
    "param1"    INTEGER NOT NULL DEFAULT 0,
    "param2"    INTEGER NOT NULL DEFAULT 0,
    "param3"    INTEGER NOT NULL DEFAULT 0,
    FOREIGN KEY("message_id") REFERENCES "ddon_system_mail"("message_id") ON DELETE CASCADE
);

Checklist:

• [x] The project compiles
• [x] The PR targets develop branch

[pacampbell]

Will wait for #369 to merge so I can use the migration functionality with this PR.