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/db/connection.py

@@ -10,15 +10,17 @@ That is the case on the CI server.
 import os
 
 import sqlalchemy as sa
+from sqlalchemy import engine as engine_mod
 from sqlalchemy import orm
 
 import urban_meal_delivery
 
 
 if os.getenv('TESTING'):
-    engine = None
-    connection = None
-    session = None
+    # Specify the types explicitly to make mypy happy.
+    engine: engine_mod.Engine = None
+    connection: engine_mod.Connection = None
+    session: orm.Session = None
 
 else:  # pragma: no cover
     engine = sa.create_engine(urban_meal_delivery.config.DATABASE_URI)