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/Disable-explosion-knockback.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Disable explosion knockback
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index a5ec0bc0e0..6a307d5dd6 100644
+index a5ec0bc0e..6a307d5dd 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 {
@@ -19,7 +19,7 @@ index a5ec0bc0e0..6a307d5dd6 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/EntityLiving.java b/src/main/java/net/minecraft/server/EntityLiving.java
-index 85567a7072..d85172e728 100644
+index d630b1be1..2212f3198 100644
 --- a/src/main/java/net/minecraft/server/EntityLiving.java
 +++ b/src/main/java/net/minecraft/server/EntityLiving.java
 @@ -0,0 +0,0 @@ public abstract class EntityLiving extends Entity {
@@ -48,7 +48,7 @@ index 85567a7072..d85172e728 100644
                  if (!this.f(damagesource)) {
                      SoundEffect soundeffect = this.getSoundDeath();
 diff --git a/src/main/java/net/minecraft/server/Explosion.java b/src/main/java/net/minecraft/server/Explosion.java
-index bcff117619..6eeff4ce59 100644
+index bcff11761..6eeff4ce5 100644
 --- a/src/main/java/net/minecraft/server/Explosion.java
 +++ b/src/main/java/net/minecraft/server/Explosion.java
 @@ -0,0 +0,0 @@ public class Explosion {