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

migrations/script.py.mako

@@ -0,0 +1,31 @@
+"""${message}.
+
+Revision: # ${up_revision} at ${create_date}
+Revises: # ${down_revision | comma,n}
+"""
+
+import os
+
+import sqlalchemy as sa
+from alembic import op
+${imports if imports else ""}
+
+from urban_meal_delivery import configuration
+
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+config = configuration.make_config('testing' if os.getenv('TESTING') else 'production')
+
+
+def upgrade():
+    """Upgrade to revision ${up_revision}."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade():
+    """Downgrade to revision ${down_revision}."""
+    ${downgrades if downgrades else "pass"}