Fail gracefully on bad DATABASE_URL

Also 'recommend' sqlite by default. Although really we will run
postgres in production.
put the test db in the test homedir
Add flask app for unsubscribing
Add notes on unsubscribes and postgres

Also remove psycopg as a hard requirement, since it's not necessary
if you run with sqlite.
Add note about the flask app
Include instructions on maual usage of unsubscribe script
Avoid hashing twice when adding unsubscribes
s/exit/sys.exit