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/Configurable-top-of-nether-void-damage.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Configurable top of nether void damage
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index 1ed58f4bba..a797a57671 100644
+index 1ed58f4bb..a797a5767 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 @@ -0,0 +0,0 @@ public class PaperWorldConfig {
@@ -29,7 +29,7 @@ index 1ed58f4bba..a797a57671 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
-index de57289fee..fef69c7c3f 100644
+index de57289fe..fef69c7c3 100644
 --- a/src/main/java/net/minecraft/server/Entity.java
 +++ b/src/main/java/net/minecraft/server/Entity.java
 @@ -0,0 +0,0 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
@@ -75,7 +75,7 @@ index de57289fee..fef69c7c3f 100644
          this.die();
      }
 diff --git a/src/main/java/net/minecraft/server/EntityMinecartAbstract.java b/src/main/java/net/minecraft/server/EntityMinecartAbstract.java
-index 4d2ef9a02b..6fc332dbff 100644
+index 4d2ef9a02..6fc332dbf 100644
 --- a/src/main/java/net/minecraft/server/EntityMinecartAbstract.java
 +++ b/src/main/java/net/minecraft/server/EntityMinecartAbstract.java
 @@ -0,0 +0,0 @@ public abstract class EntityMinecartAbstract extends Entity {