Feat: Persistent Storage with PostgreSQL

[Nfrederiksen]

feature to add support for PSQL as persistent storage.

Overview of changes:

• Removed unused files
• Added migrations folder to set up psql databases and tables
• Added 2 new docker-compose files. docker-compose-postgresql.yml for setting up containers for PSQL server and Adserver. docker-compose-testing.yml for setting up end-2-end API testing with a psql database as storage. Use it with:
  docker-compose -f docker-compose-testing.yml up --build --abort-on-container-exit
• db-adapters now implement their own _Paginator()
• Session got new functions... .toJSON() , .fromJSON(obj), & .toObject()
• Added possibility for EventTracker class to initialize with pre-existing list. (needed for .fromJSON(obj))
• All db-adapters shall return Session Class objects. Turning them into jsons occur later at the routes.
• Added a storage.js that selects what db-adapter to use based on ENV

[birme]

Whenever you have the time @Nfrederiksen could you go through the conflicts?