add 'success' optional parameter to set-alert in update nav. menu.

I think I will need to swap out the string value 'success' with a
constant so things aren't as stringly-typed as this. With that said,
this is just a quick proof-of-concept. I can keep this around until I
build out this updated feature some more.

src/web.lisp

@@ -235,7 +235,7 @@
 			;; Authorised
 			(progn
 			  (nera:update-nav-menu (cdr (assoc "NAV-MENU" _parsed :test #'string=)))
-			  (utils:set-alert "Nav. menu updated.")
+			  (utils:set-alert "Nav. menu updated." "success")
 			  (redirect "/site-settings"))
 			;; Not Authorised
 			(progn