Add sql database and tables for user and session

[sdankel]

• Adds a script, start_local_db.sh to start a local postgres docker container
• Adds a .env file with the variables for local development (connecting to postgres)
  From what I can tell, these are the correct env variable names for the deployed environments as well. When deployed, the deployed environment variables will take precedence over the ones in .env so it should all work.
• Adds diesel ORM along with 2 migrations to create the users and sessions tables
• Adds a connection pool for the database to the rocker server
• Runs the SQL migrations when the server starts up, using run_pending_migrations
• Switches the login and session APIs to use the SQL tables rather than the in memory store
• Adds integration tests for the operations writing/reading from postgres

Note: in order to get diesel working on my machine, I had to run brew install libpq. Following this issue, cargo correctly link the directory for libpq for mac arm (it's working for me now). diesel-rs/diesel#2211

[JoshuaBatty]

Looks nice a clean. Will try and get postgres setup on my local machine so I can test these locally in the future.