Deprecate Arena `arenaName()` method.

Conversely to how the "lowercase name" of ArenaClass was deprecated in
favor of the slug, the "arena name" method here is deprecated in favor
of the "config name" of arenas. Instead of trying to pretty up an arena
name (which is doomed to fail), we just use the "config name" for pretty
printing instead, and start using slugs elsewhere.

This should give way to a much better experience with multi-word arena
names as well.

src/main/java/com/garbagemule/MobArena/framework/Arena.java

@@ -221,6 +221,10 @@ public interface Arena
 
     String configName();
 
+    /**
+     * @deprecated use {@link #configName()} instead
+     */
+    @Deprecated
     String arenaName();
 
     String getSlug();