Bug 16818: External auth redirect broken under Plack

Bug 16818: External auth redirect broken under Plack

Test plan:

0) Have either CAS or Shibboleth authentication enabled under Plack.
1) Hover over the authentication link on the staff client or OPAC, and
   notice that it has either '.../opac/...' or '.../intranet/...' instead
   of '.../cgi-bin/koha/...'. (This will be a complete dealbreaker for CAS
   authentication.)
2) Apply patch.
3) Check links again; they should now have the correct paths.

Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Did not test CAS or Shibboleth, but no regression found.

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit d475dae77313e8f0ee11146ce824d019351cbc5f)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>