Constrain serial.issuance.holding_code to be valid JSON or NULL

Constrain serial.issuance.holding_code to be valid JSON or NULL

*NOTE* when merging to master and rel_2_2, you'll need to replace XXXX
not only in the upgrade script but in the version-upgrade script as well!

This avoids serial.materialize_holding_code() failing on bad data.  The
upgrade script will actually throw away bad values for
serial.issuance.holding_code.  This is no real loss, since bad data
there prevents any serials functions around the row in question from
working properly anyway.

This problem was reported by Martha Driscoll and Ben Shum.

*Also* put a couple of changes missed from the 0700 upgrade script into
210.schema.serials.sql.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>