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/0082-Add-PlayerUseUnknownEntityEvent.patch

@@ -1,43 +0,0 @@
-From 2e277508847c335f51c14a2bb61758099d1bdbcb Mon Sep 17 00:00:00 2001
-From: Jedediah Smith <jedediah@silencegreys.com>
-Date: Sat, 2 Apr 2016 05:09:16 -0400
-Subject: [PATCH] Add PlayerUseUnknownEntityEvent
-
-
-diff --git a/src/main/java/net/minecraft/server/PacketPlayInUseEntity.java b/src/main/java/net/minecraft/server/PacketPlayInUseEntity.java
-index 680adbdeb0..3f7697b39c 100644
---- a/src/main/java/net/minecraft/server/PacketPlayInUseEntity.java
-+++ b/src/main/java/net/minecraft/server/PacketPlayInUseEntity.java
-@@ -5,7 +5,7 @@ import javax.annotation.Nullable;
- 
- public class PacketPlayInUseEntity implements Packet<PacketListenerPlayIn> {
- 
--    private int a;
-+    private int a; public int getEntityId() { return this.a; } // Paper - add accessor
-     private PacketPlayInUseEntity.EnumEntityUseAction action;
-     private Vec3D c;
-     private EnumHand d;
-diff --git a/src/main/java/net/minecraft/server/PlayerConnection.java b/src/main/java/net/minecraft/server/PlayerConnection.java
-index b7c8cab9ea..ce98f690e8 100644
---- a/src/main/java/net/minecraft/server/PlayerConnection.java
-+++ b/src/main/java/net/minecraft/server/PlayerConnection.java
-@@ -1922,6 +1922,16 @@ public class PlayerConnection implements PacketListenerPlayIn {
-                 }
-             }
-         }
-+        // Paper start - fire event
-+        else {
-+            this.server.getPluginManager().callEvent(new com.destroystokyo.paper.event.player.PlayerUseUnknownEntityEvent(
-+                this.getPlayer(),
-+                packetplayinuseentity.getEntityId(),
-+                packetplayinuseentity.b() == PacketPlayInUseEntity.EnumEntityUseAction.ATTACK,
-+                packetplayinuseentity.c() == EnumHand.MAIN_HAND ? EquipmentSlot.HAND : EquipmentSlot.OFF_HAND
-+            ));
-+        }
-+        // Paper end
- 
-     }
- 
--- 
-2.21.0
-