Make label_class on any new call numbers default to org_unit setting

Make label_class on any new call numbers default to org_unit setting

Evergreen 2.0 added a label_class column to the call number table with
one major purpose being the generation of correct sort keys. You can
also specify a default label class as an org unit setting. However:

1) There are no interface elements for setting the label class of an
individual call number.

2) The default setting from the actor.org_unit_setting
'cat.default_classification_scheme' value, if set, is not consulted
(that is, not set in the call number table) when new call numbers are
created.

These two facts together greatly reduce the utility of this very
valuable feature, as all new call numbers end up in the 'Generic' class.

While #1 has been addressed in 2.1+, there is still work to be done in
setting this class on import. Also, which interface parts (if any) make
it back to 2.0 is subject to debate.

This commit addresses #2.

See lp #787150.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Dan Scott <dan@coffeecode.net>