Adding robots.txt override via admin control panel

This is a requested low priority feature in #418 - I created the changes
to follow similarly to how the Instance Description is handled.

packages/backend/src/models/Meta.ts

@@ -599,6 +599,11 @@ export class MiMeta {
 	})
 	public enableAchievements: boolean;
 
+	@Column('varchar', {
+		length: 2048, nullable: true,
+	})
+	public robotsTxt: string | null;
+
 	@Column('jsonb', {
 		default: { },
 	})