Fix arena sign event handler priority.

The previous change to this class introduced a _mutating_ method call,
which would be fine if it wasn't for the fact that the handler listened
on `MONITOR` priority prior to this commit. But no more! We're knocking
it down a notch to better comply with event handler best practices.

While this change doesn't _fix_ #787, it does allow @molor to implement
a hackaround that should keep things in check until we can bump up to a
later API version and use the new sign edit event to fix this particular
issue the right way.

src/main/java/com/garbagemule/MobArena/signs/HandlesSignClicks.java

@@ -29,7 +29,7 @@ class HandlesSignClicks implements Listener {
         this.cooldowns = new HashMap<>();
     }
 
-    @EventHandler(priority = EventPriority.MONITOR)
+    @EventHandler(priority = EventPriority.HIGHEST)
     public void on(PlayerInteractEvent event) {
         Block block = event.getClickedBlock();
         if (block == null) {