Make Grid.gridify() use only pickup addresses

- ensure a `Restaurant` only has one unique `Order.pickup_address`
- rework `Grid.gridify()` so that only pickup addresses are assigned
  into `Pixel`s
- include database migrations to ensure the data adhere to these
  tighter constraints

src/urban_meal_delivery/console/gridify.py

@@ -8,7 +8,7 @@ from urban_meal_delivery.console import decorators
 
 
 @click.command()
-@decorators.db_revision('888e352d7526')
+@decorators.db_revision('e86290e7305e')
 def gridify() -> None:  # pragma: no cover  note:b1f68d24
     """Create grids for all cities.