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/Add-configurable-portal-search-radius.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Add configurable portal search radius
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index be91b11242..320fd07c62 100644
+index be91b1124..320fd07c6 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 be91b11242..320fd07c62 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/PortalTravelAgent.java b/src/main/java/net/minecraft/server/PortalTravelAgent.java
-index 13f80ac832..02e5487efb 100644
+index 576fd6172..524dc7daa 100644
 --- a/src/main/java/net/minecraft/server/PortalTravelAgent.java
 +++ b/src/main/java/net/minecraft/server/PortalTravelAgent.java
 @@ -0,0 +0,0 @@ public class PortalTravelAgent {