Adds UUID fetching on player join. See #12

Whenever a player joins, their names will be checked against a map containing all names which need to be migrated to UUID. All portals the player has created which still use the player name will be updated.

src/main/java/net/knarcraft/stargate/portal/PortalOwner.java

@@ -43,6 +43,22 @@ public class PortalOwner {
         return ownerUUID;
     }
 
+    /**
+     * Sets the unique id for a portal owner without one
+     *
+     * <p>This method is only meant to be used to set the unique id for an owner without one. If the owner already has
+     * an unique id, an exception will be thrown.</p>
+     *
+     * @param uniqueId <p>The new unique id for the portal owner</p>
+     */
+    public void setUUID(UUID uniqueId) {
+        if (ownerUUID == null) {
+            ownerUUID = uniqueId;
+        } else {
+            throw new IllegalArgumentException("An existing UUID cannot be overwritten.");
+        }
+    }
+
     /**
      * Gets the name of this owner
      *