Support postgres 9.6

This is a requirement for balsac deployment (until an update from arbutus DB team).

We are stuck to Postgres 9.6 on their deployment with the following extensions:

   Name    | Version |   Schema   |                             Description
-----------+---------+------------+---------------------------------------------------------------------
 plpgsql   | 1.0     | pg_catalog | PL/pgSQL procedural language
 postgis   | 3.0.2   | public     | PostGIS geometry, geography, and raster spatial types and functions
 uuid-ossp | 1.1     | public     | generate universally unique identifiers (UUIDs)

The GENERATED ALWAYS columsn are not available until postgres 12. We could fix that by squashing the DB migrations for the images app