move to postgres.

[olaven]

• set up postgres libraries
• update interfaces
• remove mongo
• implement one schema per service (instead of one database) (check out this)
• delete old mongo setup in cloud
• configure postgres in DO
• update env variables for prod with postgres -> Create full CI deployment with Terraform/Actions #91
• possible to run postgres in memory when testing? -> In memory instance of postgres when running tests. #92
• fix all tests before merging

Many of these should probably be separate cards.