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/conftest.py

@@ -0,0 +1,10 @@
+"""Fixture for testing the CLI scripts."""
+
+import pytest
+from click import testing as click_testing
+
+
+@pytest.fixture
+def cli() -> click_testing.CliRunner:
+    """Initialize Click's CLI Test Runner."""
+    return click_testing.CliRunner()