Bug 30658: (bug 29496 follow-up) CheckMandatorySubfields don't work properly with...

Bug 30658: (bug 29496 follow-up) CheckMandatorySubfields don't work properly with select field in serials-edit.tt for Supplemental issue

CheckMandatorySubfields use the class "input_marceditor" but in file
serials-edit.tt this class is not set for select input in Supplemental
issue form. In consequence if a select field is set as mandatory, it is
detected as missing even if it is filed and so you can't submit the form
and receive the new supplemantal issue.

Test plan:
1- Create (or find) a subscription for a biblio record and select the
   option "Create an item record when receiving this serial"
2- Be sure to have at least one mandatory subfield that is filed with a
   select input in the framework used by the biblio record. (ex: 952$a,
   952$b or 952$c).
3- From the subscription-detail page click on "Receive"
4- In the lower part--Supplemental issue--fill the suppelemant details,
   change the status to "Arrived" and fill the item form that appears.
5- Click on "Save"
6- Check that an error box appear with the message " Form not submitted
   because of the following problem(s) 1 mandatory fields empty
   (highlighted)" (the number can be different according to the number of
   concerned subfields)
7- Apply the patch
8- Repeat step 3 to 5
9- Check that no error appear and that your item has been created

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>