mbsync: put extra config at the beginning

If it is at the end it will just end up applying to the last defined
section.

Fixes #748

(cherry picked from commit c3520bfa52)

modules/programs/mbsync.nix

@@ -173,9 +173,9 @@ in
       in
         concatStringsSep "\n" (
           [ "# Generated by Home Manager.\n" ]
+          ++ optional (cfg.extraConfig != "") cfg.extraConfig
           ++ accountsConfig
           ++ groupsConfig
-          ++ optional (cfg.extraConfig != "") cfg.extraConfig
         ) + "\n";
 
     home.activation.createMaildir =