zsh: move aliases definitions after initExtra

Same motivation as in https://github.com/NixOS/nixpkgs/pull/28378.
zsh.initExtra parameter can be used by external modules which can
redefine user aliases. This change will give user-defined aliases
the highest priority.

modules/misc/news.nix

@@ -107,6 +107,19 @@ in
 
   config = {
     news.entries = [
+      {
+        time = "2017-09-12T13:11:48+00:00";
+        condition = (
+          config.programs.zsh.enable &&
+          config.programs.zsh.shellAliases != {}
+        );
+        message = ''
+          Aliases defined in 'programs.zsh.shellAliases'
+          are now have the highest priority. Such aliases will
+          not be redefined by the code in 'programs.zsh.initExtra'
+          or any external plugins.
+        '';
+      }
       {
         time = "2017-09-10T22:15:19+00:00";
         condition = config.programs.zsh.enable;

modules/programs/zsh.nix

@@ -241,9 +241,9 @@ in
           source "$HOME/.zsh/plugins/${plugin.name}/${plugin.file}"
         '') cfg.plugins)}
 
-        ${aliasesStr}
-
         ${cfg.initExtra}
+
+        ${aliasesStr}
       '';
     })
     (mkIf cfg.oh-my-zsh.enable {