Add database migrations

- use Alembic to migrate the PostgreSQL database
  + create initial migration script to set up the database,
    as an alternative to db.Base.metadata.create_all()
  + integrate Alembic into the test suite; the db_engine fixture
    now has two modes:
    * create the latest version of tables all at once
    * invoke `alembic upgrade head`
    => the "e2e" tests are all run twice, once in each mode; this
       ensures that the migration scripts re-create the same database
       schema as db.Base.metadata.create_all() would
    * in both modes, a temporary PostgreSQL schema is used to create the
      tables in
    => could now run "e2e" tests against production database and still
       have isolation
- make the configuration module public (to be used by Alembic)
- adjust linting rules for Alembic

tests/conftest.py

@@ -0,0 +1,12 @@
+"""Utils for testing the entire package."""
+
+import os
+
+from urban_meal_delivery import config
+
+
+if not os.getenv('TESTING'):
+    raise RuntimeError('Tests must be executed with TESTING set in the environment')
+
+if not config.TESTING:
+    raise RuntimeError('The testing configuration was not loaded')