Only consider main entry headings for bib overlay

Restrict the authority fields we will pull overlay data from to those
that do not subordinate themselves to other authority fields.  That
is, skip see-from/see-also/etc for bib overlay.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>
Fix a data destruction bug when authorities are updated.

Modify the two-argument form of vandelay.merge_record_xml to return
the target_marc when the add_rule, preserve_rule, replace_rule and
strip_rule are all empty.

This method is used by authority.propagate_changes and would replace
the bibliographic record's marc with the overlay template when the
above would happen. By returning the biblographic record entry's own
marc in that case, we prevent data destruction.

Additionally, protect bib and authority template overlay functions
in the same manner.

Commit includes an upgrade script for this change as well as
for the previous commit's authority.generate_overlay_template fix.

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>
Signed-off-by: Mike Rylander <mrylander@gmail.com>