Implements #27 among other things

It was found that the Spigot API's methods for cancelling player collisions won't work without changing the scoreboard. Because of that, the normal option has been disabled. The invisibility option has also been removed, as that's a bad idea if players can still push each-other.
The toggle player option which is implemented in this commit does disable player collision, so that's the only working way right now.

A potential ConcurrentModificationException has been fixed.
The parkourCheckpoint command has been removed, as the functionality is now available through the API.

src/main/java/net/knarcraft/minigames/arena/PlayerVisibilityManager.java

@@ -0,0 +1,95 @@
+package net.knarcraft.minigames.arena;
+
+import net.knarcraft.minigames.MiniGames;
+import org.bukkit.Bukkit;
+import org.bukkit.entity.Player;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.UUID;
+
+/**
+ * A manager for keeping track of which players have set other players as hidden
+ */
+public class PlayerVisibilityManager {
+
+    private final Set<UUID> hidingEnabledFor = new HashSet<>();
+
+    /**
+     * Toggles whether players should be hidden for the player with the given id
+     *
+     * @param player <p>The the player to update</p>
+     */
+    public void toggleHidePlayers(@NotNull ArenaPlayerRegistry<?> playerRegistry, @NotNull Player player) {
+        if (hidingEnabledFor.contains(player.getUniqueId())) {
+            hidingEnabledFor.remove(player.getUniqueId());
+            // Make all other players visible again
+            changeVisibilityFor(playerRegistry, player, false);
+        } else {
+            hidingEnabledFor.add(player.getUniqueId());
+            // Make all other players hidden
+            changeVisibilityFor(playerRegistry, player, true);
+        }
+
+    }
+
+    /**
+     * Updates which players are seen as hidden
+     *
+     * @param playerRegistry <p>The registry containing all playing players</p>
+     * @param player         <p>The player that joined the arena</p>
+     */
+    public void updateHiddenPlayers(@NotNull ArenaPlayerRegistry<?> playerRegistry, @NotNull Player player) {
+        boolean hideForPlayer = hidingEnabledFor.contains(player.getUniqueId());
+        for (UUID playerId : playerRegistry.getPlayingPlayers()) {
+            Player otherPlayer = Bukkit.getPlayer(playerId);
+            if (otherPlayer == null) {
+                continue;
+            }
+            // Hide the arena player from the newly joined player
+            if (hideForPlayer) {
+                player.hidePlayer(MiniGames.getInstance(), otherPlayer);
+            }
+            // Hide the newly joined player from this player
+            if (hidingEnabledFor.contains(playerId)) {
+                otherPlayer.hidePlayer(MiniGames.getInstance(), player);
+            }
+        }
+    }
+
+    /**
+     * Makes all players visible to the given player
+     *
+     * @param player <p>The player to update visibility for</p>
+     */
+    public void showPlayersFor(@NotNull Player player) {
+        for (Player otherPlayer : Bukkit.getOnlinePlayers()) {
+            player.showPlayer(MiniGames.getInstance(), otherPlayer);
+            otherPlayer.showPlayer(MiniGames.getInstance(), player);
+        }
+    }
+
+    /**
+     * Changes whether the given player can see the other players in the arena
+     *
+     * @param playerRegistry <p>The player registry containing other players</p>
+     * @param player         <p>The player to change the visibility for</p>
+     * @param hide           <p>Whether to hide the players or show the players</p>
+     */
+    private void changeVisibilityFor(@NotNull ArenaPlayerRegistry<?> playerRegistry, @NotNull Player player, boolean hide) {
+        for (UUID playerId : playerRegistry.getPlayingPlayers()) {
+            Player otherPlayer = Bukkit.getPlayer(playerId);
+            if (otherPlayer == null) {
+                continue;
+            }
+
+            if (hide) {
+                player.hidePlayer(MiniGames.getInstance(), otherPlayer);
+            } else {
+                player.showPlayer(MiniGames.getInstance(), otherPlayer);
+            }
+        }
+    }
+
+}