add username check to /sign-up defroute.

Just an extra check in amongst what's already there. If the username
the new sign-up attempt has entered matched one already in the
database, a message is relayed stating as much and redirects back to
the sign-up page.

src/web.lisp

@@ -84,6 +84,9 @@
        (lack.request:request-body-parameters ningle:*request*))
     (cond ((not (string= authenticity-token (auth:csrf-token)))
 	   `(,+forbidden+ (:content-type "text/plain") ("Denied")))
+	  ((not (null (nera:get-user username)))
+	   (utils:set-alert "Username already taken.")
+	   (redirect "/sign-up"))
 	  ((not (string= password password-check))
 	   (utils:set-alert "Passwords don't match.")
 	   (redirect "/sign-up"))