Bug 36141: Add classes to CAS text on OPAC login page

Bug 36141: Add classes to CAS text on OPAC login page

This enhancement makes it easier for libraries to change the CAS-related messages on the OPAC login page.

It moved the invalid CAS login message above the CAS loging heading,
like for Shibboleth login.

Test plan :
1) Enable system preference 'casAuthentication'
2) Restart all caches (restart_all in koha-testing-docker)
3) Go to OPAC, logged out
4) Click on 'Log in to your account'
5) In the staff interface, edit the OPACUserJS system preference. Add the following JS and Save:
   $(".cas_invalid").text("Test changing the invalid CAS login message.");
   $(".cas_title").text("Test changing the CAS login heading.");
   $(".cas_url").text("Test changing the CAS account link text.");
   $(".cas_url").after(' <i class="fa fa-globe" aria-hidden="true"></i>');
6) Refresh the OPAC and confirm the text changes to reflect your JS.

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>