Performs a lot of changes. Mostly implements #3

Properly and persistently stores which players have beaten which arenas
Adds a hopefully fully functional DropperArenaGroup class
Stores arena groups in the arena handler
Adds a lookup map to improve performance of getting arena by name
Adds saving and loading of groups
Adds all necessary checks for whether players have beaten the required dropper arenas before joining
Removes stage from arenas
Adds a test to make sure changing the order of arenas in a group works as intended

src/test/java/net/knarcraft/dropper/arena/DropperArenaGroupTest.java

@@ -0,0 +1,50 @@
+package net.knarcraft.dropper.arena;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+
+/**
+ * Tests for arena dropper groups
+ */
+public class DropperArenaGroupTest {
+
+    @Test
+    public void swapTest() {
+        /*
+           This test makes sure the order of arenas is as expected when the arenas are added to a group. It also makes
+           sure that swapping two items works as expected.
+         */
+
+        DropperArenaGroup arenaGroup = new DropperArenaGroup("test");
+        UUID arena1Id = UUID.randomUUID();
+        UUID arena2Id = UUID.randomUUID();
+        UUID arena3Id = UUID.randomUUID();
+        UUID arena4Id = UUID.randomUUID();
+
+        arenaGroup.addArena(arena1Id);
+        arenaGroup.addArena(arena2Id);
+        arenaGroup.addArena(arena3Id);
+        arenaGroup.addArena(arena4Id);
+
+        List<UUID> initialOrder = new ArrayList<>();
+        initialOrder.add(arena1Id);
+        initialOrder.add(arena2Id);
+        initialOrder.add(arena3Id);
+        initialOrder.add(arena4Id);
+        Assertions.assertEquals(initialOrder, arenaGroup.getArenas());
+
+        arenaGroup.swapArenas(1, 3);
+
+        List<UUID> swapped = new ArrayList<>();
+        swapped.add(arena1Id);
+        swapped.add(arena4Id);
+        swapped.add(arena3Id);
+        swapped.add(arena2Id);
+        Assertions.assertEquals(swapped, arenaGroup.getArenas());
+    }
+
+}