Public sign up should not be available, if external user management is ON

Steps to replicate:

1. Enable External user management and the Public mode from the JIRA admin interface > General Configuration.
2. Log out .
3. Navigate to the dashboard page (http://localhost:8080/secure/Dashboard.jspa) DOSE NOT display the "Sing up" option.
   However, if you enter an incorrect user, you be redirected to the log in page at http://localhost:8080/login.jsp which DOES displays the "sign up" link.

We should make access to the sign up consistent all across JIRA.
Also, since the sign up is controlled by the mode in General Configuration, it doesn't make sense to restrict it through the external management. Unless, we be also checking if JIRA is able to create new users in the remote repository.