LP#1037553: bibliographic record source now copied to 901$s

LP#1037553: bibliographic record source now copied to 901$s

If a bibliographic record has a source set, the name of that source
is now copied to the 901$s whenever the record is created or updated.
This allows the source to be used for record matching and MARC
field queries.

To test:

[1] Create or import a bibliographic record and ensure that its
    source is set to a non-null value.
[2] Inspect the record in MARC format view and verify that its
    901 field now has a subfield $s containing the name of the
    source.
[3] Use pg_prove to run the pgTAP test added by this patch
    and verify that the test passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>