auth: Always show agent login link

Previously, if the enduser registration mode were set to something other
than `public`, the agent sign-in link would not be shown. Now, the agent
sign-in link is always shown on the enduser sign-in page.
Merge pull request #1196 from greezybacon/issue/agent-login

auth: Always show agent login link

Reviewed-By: Peter Rotich <peter@osticket.com>