Bug 29006: Make GoogleOpenIDConnect options consistent in the OPAC

Bug 29006: Make GoogleOpenIDConnect options consistent in the OPAC

This patch makes modifications to OPAC templates to make the Google
login options more consistent.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- In the staff interface, enable the GoogleOpenIDConnect preference, and
  populate the GoogleOAuth2ClientID the GoogleOAuth2ClientSecret with
  values (they don't need to be valid).
- On the OPAC main page you should see a "Log in with Google" button
  above the Koha login form.
- If you click the "Log in to your account" link at the top of the page
  you should see a "Log in with Google" button in the modal window
- If you navigate directly to /cgi-bin/koha/opac-user.pl when not logged
  in you should see the same "Log in with Google" button on that page.
- In each case the Google button should point to /cgi-bin/koha/svc/auth/googleopenidconnect
- If you disable GoogleOpenIDConnect the buttons should disappear.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>