Add CLI script to gridify all cities

- reorganize `urban_meal_delivery.console` into a sub-package
- move `tests.db.conftest` fixtures into `tests.conftest`
  => some integration tests regarding CLI scripts need a database
- add `urban_meal_delivery.console.decorators.db_revision` decorator
  to ensure the database is at a certain state before a CLI script runs
- refactor the `urban_meal_delivery.db.grids.Grid.gridify()` constructor:
  - bug fix: even empty `Pixel`s end up in the database temporarily
    => create `Pixel` objects only if an `Address` is to be assigned
       to it
  - streamline code and docstring
  - add further test cases

tests/console/test_gridify.py

@@ -0,0 +1,41 @@
+"""Integration test for the `urban_meal_delivery.console.gridify` module."""
+
+import pytest
+
+import urban_meal_delivery
+from urban_meal_delivery import db
+from urban_meal_delivery.console import gridify
+
+
+@pytest.mark.db
+def test_four_pixels_with_two_addresses(
+    cli, db_session, monkeypatch, city, make_address,
+):
+    """Two `Address` objects in distinct `Pixel` objects.
+
+    This is roughly the same test case as
+    `tests.db.test_grids.test_four_pixels_with_two_addresses`.
+    The difference is that the result is written to the database.
+    """
+    # Create two `Address` objects in distinct `Pixel`s.
+    city.addresses = [
+        # One `Address` in the lower-left `Pixel`, ...
+        make_address(latitude=48.8357377, longitude=2.2517412),
+        # ... and another one in the upper-right one.
+        make_address(latitude=48.8898312, longitude=2.4357622),
+    ]
+
+    db_session.add(city)
+    db_session.commit()
+
+    side_length = max(city.total_x // 2, city.total_y // 2) + 1
+
+    # Hack the configuration regarding the grids to be created.
+    monkeypatch.setattr(urban_meal_delivery.config, 'GRID_SIDE_LENGTHS', [side_length])
+
+    result = cli.invoke(gridify.gridify)
+
+    assert result.exit_code == 0
+
+    assert db_session.query(db.Grid).count() == 1
+    assert db_session.query(db.Pixel).count() == 2