bug 2206: always have $9 for headings fields

bug 2206: always have $9 for headings fields

If a field can be linked to an authority record (i.e.,
an authtypecode is defined for subfield $a), always
include the subfield $9 in a (readonly) input element even
if it is marked hidden per the MARC framework.

Prior to this patch, a $9 marked hidden per the framework
would not be included in the form, causing the authority record
number link to not be added when choosing a heading from the authority
finder.  In other words, with BiblioAddsAuthorities OFF,
any authorized headings in bibs added via the MARC editor
would not have the subfield $9, and thus would appear
to not be used by any bibs.

Note that subfield $9 is set to be readonly, as changing the
authority number link does not currently change the
heading stored in the bib record.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>