only store one object of GameType and Ruleset when they already exists in the database

dbschema/default.esdl

@@ -19,13 +19,9 @@ module default {
     }
     required single type: GameType {
       readonly := true;
-      constraint exclusive;
-      on source delete delete target;
     }
     required single ruleset: Ruleset {
       readonly := true;
-      constraint exclusive;
-      on source delete delete target;
     }
     required winner: str {
       readonly := true;
@@ -46,7 +42,7 @@ module default {
     required is_ladder: bool {
       readonly := true;
     }
-    #constraint exclusive on ( (.name, .is_ladder) );
+    constraint exclusive on ( (.name, .is_ladder) );
   }
 
   type Ruleset {
@@ -59,7 +55,7 @@ module default {
     required settings: json {
       readonly := true;
     }
-    #constraint exclusive on ( (.name, .version, .settings) );
+    constraint exclusive on ( (.name, .version, .settings) );
   }
 
   type Snake {