Bug 20063: (bug 18811 follow-up) Allow hidden $9 subfields

Although we could argue that the user should make all $9 subfields
visible, this might be a good compromise. It selectively reverts the
change from the first patch by accepting $9 subfields without checking
the hidden flag in the framework.

Test plan:
[1] Make a subfield $9 visible for some authority field.
[2] Put a value in it somewhere in authorities.pl
[3] Make subfield $9 invisible in the auth framework.
[4] Open and save the adjusted authority record.
[5] Check the marcxml of this record.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

authorities/authorities.pl

@@ -404,7 +404,7 @@ sub build_tabs {
                             $subfield = '@';
                         }
                         next if ( $tagslib->{$tag}->{$subfield}->{tab} ne $tabloop );
-                        next if $tagslib->{$tag}->{$subfield}->{hidden};
+                        next if $tagslib->{$tag}->{$subfield}->{hidden} && $subfield ne '9';
                         push(
                             @subfields_data,
                             &create_input(
@@ -420,7 +420,7 @@ sub build_tabs {
                             my $value    = $subfields[$subfieldcount][1];
                             next if ( length $subfield != 1 );
                             next if ( $tagslib->{$tag}->{$subfield}->{tab} ne $tabloop );
-                            next if $tagslib->{$tag}->{$subfield}->{hidden};
+                            next if $tagslib->{$tag}->{$subfield}->{hidden} && $subfield ne '9';
                             push(
                                 @subfields_data,
                                 &create_input(
@@ -437,7 +437,7 @@ sub build_tabs {
                         next if ( length $subfield != 1 );
                         next if ( $tagslib->{$tag}->{$subfield}->{tab} ne $tabloop );
                         next if ( $tag < 10 );
-                        next if $tagslib->{$tag}->{$subfield}->{hidden};
+                        next if $tagslib->{$tag}->{$subfield}->{hidden} && $subfield ne '9';
                         next if ( defined( $field->subfield($subfield) ) );
                         push(
                             @subfields_data,
@@ -476,7 +476,7 @@ sub build_tabs {
                 my @subfields_data;
                 foreach my $subfield ( sort( keys %{ $tagslib->{$tag} } ) ) {
                     next if ( length $subfield != 1 );
-                    next if $tagslib->{$tag}->{$subfield}->{hidden};
+                    next if $tagslib->{$tag}->{$subfield}->{hidden} && $subfield ne '9';
                     next if ( $tagslib->{$tag}->{$subfield}->{tab} ne $tabloop );
                     push(
                         @subfields_data,