Move version command update checking to the implementation

This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()

It also makes it easier for us to organize things like the version
history feature.

Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.

Spigot-Server-Patches/0247-Implement-World.getEntity-UUID-API.patch

@@ -0,0 +1,29 @@
+From 4d9a417496995c21613e5e027553b0354bf120a5 Mon Sep 17 00:00:00 2001
+From: Brokkonaut <hannos17@gmx.de>
+Date: Tue, 3 Jul 2018 16:08:14 +0200
+Subject: [PATCH] Implement World.getEntity(UUID) API
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+index 83a8f083a..4c67f071b 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+@@ -1107,6 +1107,15 @@ public class CraftWorld implements World {
+         return list;
+     }
+ 
++    // Paper start - getEntity by UUID API
++    @Override
++    public Entity getEntity(UUID uuid) {
++        Validate.notNull(uuid, "UUID cannot be null");
++        net.minecraft.server.Entity entity = world.getEntity(uuid);
++        return entity == null ? null : entity.getBukkitEntity();
++    }
++    // Paper end
++
+     @Override
+     public void save() {
+         this.server.checkSaveState();
+-- 
+2.21.0
+