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/__init__.py

@@ -0,0 +1,5 @@
+"""Test the CLI scripts in the urban-meal-delivery package.
+
+Some tests require a database. Therefore, the corresponding code is excluded
+from coverage reporting with "pragma: no cover" (grep:b1f68d24).
+"""