Bug 12046: Authentication using CAS - bootstrap

Bug 12046: Authentication using CAS - bootstrap

At the OPAC, if the bootstrap theme is used, the modal
dialog does not allow to use the CAS authentication.

This patch proposes, in case CAS is enabled, to
redirect to the opac-user.pl page (like prog theme).
This is because the popup content should stay small
(thinking about mobile surf), and that CAS url is
actually only computed in opac-user (see C4/Auth.pm)
and it would not be performant to compute it in all
pages.

Test plan:
- set syspref opacthemes to bootstrap
- enable the casAuthentication syspref
- fill the casServerUrl syspref with something like:
  https://localhost:8443/cas
- go on the opac home page
- click on "Log in to your account" link (top right)
=> You go to cgi-bin/koha/opac-user.pl page where you
  see the cas link
- disable the casAuthentication syspref
- go on the opac home page
- click on "Log in to your account" link (top right)
=> You see a popup for login and password

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Normal login still works with/without CAS.
Passes all tests and QA script.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 4bc46dbbcb93ca43b2fd138cf65f51893978b3ad)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>