Bug 33766: Fix ambiguous "Login" field

Bug 33766: Fix ambiguous "Login" field

This patch introduces a new syspref 'OPACLoginLabelTextContent' which can be set
to the following values
- Cardnumber
- Username
- Cardnumber or Username

To test:
- Check that the label for
    - The masthead inc
    - The opac-auth template
    - The opac-main template
    - The opac-password-recovery template
    - The opac-registration-confirmation template
    - The opac-reset-password template
    - The sco-main template
  reflect the changes you made to the syspref

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>