Bug 29496: CheckMandatorySubfields don't work properly with select field in serials...

Bug 29496: CheckMandatorySubfields don't work properly with select field in serials-edit.tt

Since one of the patches of BZ 27526 (Bug 27526: Fix mandatory and
important checks), CheckMandatorySubfields use the class
"input_marceditor" but in file serials-edit.tt this class is not set for
all field (it is present on text input but not on select input) 5
9- Check that no error appear and that your item has been created.

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 serial.

Test plan:
0- Be sure to be in a version of koha where the patch that introduces
   the bug is present (it is present in master since Jul 8 2021 (it is
   present in 21.06.00.046) and will be pushed in 21.11.00)
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: 995$b,
   995$c or 995$e in unimarc; 952$a, 952$b or 952$c in marc21)
3- From the subscription-detail page click on "Receive"
4- 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

JD amended patch: remove comma to separate classes

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>