Fix the navigation links for NON_STAFF to create new ticket if HELPDESK_...

This code change fix the following problem:

When HELPDESK_ALLOW_NON_STAFF_TICKET_UPDATE is set to True, a NON-STAFF user goes to helpdesk (helpdesk/) will be redirected to the dashboard (helpdesk/dashboard/). But the "Submit A Ticket" nav link points to "helpdesk/", which leads the users to dashboard too. Then there is no link for the user to create a new ticket.

This fix corrects the "Submit A Ticket" link and provides another link to go back to the dashboard for non-staff users.