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

setup.cfg

@@ -102,6 +102,20 @@ per-file-ignores =
     docs/conf.py:
         # Allow shadowing built-ins and reading __*__ variables.
         WPS125,WPS609,
+    migrations/env.py:
+        # Type annotations are not strictly enforced.
+        ANN0, ANN2,
+    migrations/versions/*.py:
+        # Type annotations are not strictly enforced.
+        ANN0, ANN2,
+        # File names of revisions are ok.
+        WPS114,WPS118,
+        # Revisions may have too many expressions.
+        WPS204,WPS213,
+        # No overuse of string constants (e.g., 'RESTRICT').
+        WPS226,
+        # Too many noqa's are ok.
+        WPS402,
     noxfile.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,
@@ -111,7 +125,7 @@ per-file-ignores =
         WPS213,
         # No overuse of string constants (e.g., '--version').
         WPS226,
-    src/urban_meal_delivery/_config.py:
+    src/urban_meal_delivery/configuration.py:
         # Allow upper case class variables within classes.
         WPS115,
         # Numbers are normal in config files.
@@ -255,5 +269,7 @@ addopts =
     --strict-markers
 cache_dir = .cache/pytest
 console_output_style = count
+env =
+    TESTING=true
 markers =
     e2e: integration tests, inlc., for example, tests touching a database