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/0166-Implement-ensureServerConversions-API.patch

@@ -1,26 +0,0 @@
-From 8c1e731358dc64c98eadcd0ad88ebae60fdf9339 Mon Sep 17 00:00:00 2001
-From: Aikar <aikar@aikar.co>
-Date: Wed, 4 May 2016 22:43:12 -0400
-Subject: [PATCH] Implement ensureServerConversions API
-
-This will take a Bukkit ItemStack and run it through any conversions a server process would perform on it,
-to ensure it meets latest minecraft expectations.
-
-diff --git a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemFactory.java b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemFactory.java
-index a4320f8446..af1db68aa7 100644
---- a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemFactory.java
-+++ b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemFactory.java
-@@ -321,4 +321,10 @@ public final class CraftItemFactory implements ItemFactory {
-     public Material updateMaterial(ItemMeta meta, Material material) throws IllegalArgumentException {
-         return ((CraftMetaItem) meta).updateMaterial(material);
-     }
-+    // Paper start
-+    @Override
-+    public ItemStack ensureServerConversions(ItemStack item) {
-+        return CraftItemStack.asCraftMirror(CraftItemStack.asNMSCopy(item));
-+    }
-+    // Paper end
- }
--- 
-2.21.0
-