Add an env variable to choose the Database Schema

[stondini]

When it comes to use Postgres as database, it could be great to be able to specify the target schema where to create the tables.
An optional DB_SCHEMA variable defaulting to public may be useful.

The TypeORM PostgresConnectionOptions interface defines a schema property to fulfil this purpose.

[brunobuddy]

Closing this out. It's from Manifest's old backend-as-a-service days, which we don't work on anymore since the pivot. See #742 for where things are now.