Bug 31619: add title back to list of OPACSuggestionMandatoryFields options

Bug 31619: add title back to list of OPACSuggestionMandatoryFields options

This patch fixes a regression introduced by bug 29695 that prevented
setting the title as a required field when setting the
OPACSuggestionMandatoryFields to a non-default value.

To test:

[1] Set OPACSuggestionMandatoryFields in the system preferences
    editor and note that 'title' is not listed in the modal
    that is displayed.
[2] Apply the patch.
[3] Revist the system preference editor and note that 'title'
    is now displayed as an option.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>