18.09: Jackrabbit -> Jellyfish

See eb0fa09232 and github discussion on that.
My misgiving about the t-shirts was refuted.
I don't think this flip may negatively affect anyone.

nixos/doc/manual/release-notes/rl-1809.xml

@@ -4,7 +4,7 @@
          version="5.0"
          xml:id="sec-release-18.09">
 
-<title>Release 18.09 (“Jackrabbit”, 2018/09/??)</title>
+<title>Release 18.09 (“Jellyfish”, 2018/09/??)</title>
 
 <section xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"

nixos/modules/misc/version.nix

@@ -85,8 +85,8 @@ in
       revision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
       versionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
 
-      # Note: code names must only increase in alphabetical order.
-      codeName = "Jackrabbit";
+      # Note: the first letter is bumped on every release.  It's an animal.
+      codeName = "Jellyfish";
     };
 
     # Generate /etc/os-release.  See