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/Optimize-Server-World-Map.patch

@@ -21,7 +21,7 @@ known NMS used methods, but we can add more if naughty plugins are found later.
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldMap.java b/src/main/java/com/destroystokyo/paper/PaperWorldMap.java
 new file mode 100644
-index 0000000000..6bb2f98b45
+index 000000000..6bb2f98b4
 --- /dev/null
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldMap.java
 @@ -0,0 +0,0 @@
@@ -217,7 +217,7 @@ index 0000000000..6bb2f98b45
 +    }
 +}
 diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
-index 0209330ad4..0a02dece68 100644
+index 74cdf4945..2bd1e7370 100644
 --- a/src/main/java/net/minecraft/server/MinecraftServer.java
 +++ b/src/main/java/net/minecraft/server/MinecraftServer.java
 @@ -0,0 +0,0 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas