Prototype Postgres as backend database for ToyNet Flask

[takakonishimura]

Today we use SQLite as the choice backend database for ToyNet's flask application both in local development and in production. Given we run on ephemera docker containers deployed to ECS, this means we cannot have persistent storage when ToyNet's flask pod rolls and the database is recreated empty. In this ticket, we explore code changes required to provide Postgres its own dedicated container to run on in ECS with the intention of putting the actual Postgres data in remote storage (EBS volumes).