Bug 27526: Fix mandatory and important checks

Bug 27526: Fix mandatory and important checks

The input names have been changed from "field_value" to $kohafield.
Modifying this could have an impact in other area, where
CheckMandatorySubfields and CheckImportantSubfields are called.
Using .input_marceditor let us fix the additem.tt form and prevent to
break the other ones.
Note that the other ones are actually broken (!)

Also note that there is a typo in the error message
        alertString2 += "\n- " + "%s " + MSG_MANDATORY_FIELDS_EMPTY.format(total_errors);
There is an extra %s

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>