Use postgres with password

[normanrz]

webKnossos currently requires the postgres database to be configured with default username and password postgres:postgres. For security reasons, it would be desirable to allow arbitrary username/password combinations.
I believe JDBC and Slick already support that with the connection URL syntax jdbs:postgres://username:password@host/database. However, all adjacent scripts (e.g. ensure_db.sh) don't.