Bug 8737 - Incorrect icon at login in staff client

Added the following missing code to ensure the correct icon
is used when logged out:
   IntranetFavicon => C4::Context->preference('IntranetFavicon')
This was added into an existing $template->param() call.

Not to be confused with the koha logo on the login page, the
icon is a 16x16 pixel graphic in the browser tab. The default
is found at .../intranet-tmpl/prog/en/includes/favicon.ico.

If the "IntranetFavicon" system preference is set, it should be
used by the staff client regardless of login state. It was not
being used in the "AUTH rejected" section of Auth.pm, but the
OpacFavicon variable was being set. This explains why the
"OpacFavicon" system preference works for the OPAC client, but
not the staff client upon logout.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>