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

src/urban_meal_delivery/configuration.py

@@ -32,6 +32,8 @@ class Config:
     # time horizon, we treat it as an ad-hoc order.
     QUASI_AD_HOC_LIMIT = datetime.timedelta(minutes=45)
 
+    GRID_SIDE_LENGTHS = [707, 1000, 1414]
+
     DATABASE_URI = os.getenv('DATABASE_URI')
 
     # The PostgreSQL schema that holds the tables with the original data.