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

pom.xml

@@ -73,5 +73,11 @@
             <version>24.0.1</version>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.9.2</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 </project>