Create a unique index on authority records based on their heading, thesaurus, and...

Create a unique index on authority records based on their heading, thesaurus, and heading text

By providing a truly unique index for headings for a given thesaurus, this
index sets the stage for removing the arn_value / arn_source columns from
the database.

Sites with loaded authority records who attempt to add this index may find that
it fails due to existing duplicate entries; some suggestions for hunting down
the trouble-doers (duplicate entries) is provided in
Open-ILS/src/sql/Pg/0400.schema.unique_authority_index.sql

git-svn-id: svn://svn.open-ils.org/ILS/trunk@17704 dcc99617-32d9-48b4-a31d-7c20da2025e4