Settle open particle slot questions

Slots hold up to 9 entries, so the slot editor is one entry row plus the
bottom bar, and the server team may spawn 90 particles per tick. The
water and lava rings for taking part in any event return as reward
designs, and locked slots can be deleted from their read-only view.

LobbySystem/src/de/steamwar/lobby/particle/ParticleRequirement.java

@@ -77,6 +77,17 @@ public interface ParticleRequirement {
             return context.getUser().getTeam() != 0;
         }
     };
+    ParticleRequirement EVENT_PARTICIPATION = new ParticleRequirement() {
+        @Override
+        public String getRequirementName(Player player) {
+            return LobbySystem.getMessage().parse("PARTICLE_UNLOCKED_BY_EVENT", player);
+        }
+
+        @Override
+        public boolean test(UnlockContext context) {
+            return !context.getEvents().isEmpty();
+        }
+    }.or(SERVER_TEAM);
     ParticleRequirement EGG_HUNT_EASY = _easterEggHuntDifficulty(EggDifficulty.EASY);
     ParticleRequirement EGG_HUNT_MEDIUM = _easterEggHuntDifficulty(EggDifficulty.MEDIUM);
     ParticleRequirement EGG_HUNT_HARD = _easterEggHuntDifficulty(EggDifficulty.HARD);