Add Grid.gridify() constructor

- the purpose of this constructor method is to generate all `Pixel`s
  for a `Grid` that have at least one `Address` assigned to them
- fix missing `UniqueConstraint` in `Grid` class => it was not possible
  to create two `Grid`s with the same `.side_length` in different cities
- change the `City.viewport` property into two separate `City.southwest`
  and `City.northeast` properties; also add `City.total_x` and
  `City.total_y` properties for convenience

tests/db/test_orders.py

@@ -425,7 +425,7 @@ class TestProperties:
 
 @pytest.mark.db
 @pytest.mark.no_cover
-def test_make_random_orders(  # noqa:C901,WPS211,WPS210,WPS213,WPS231
+def test_make_random_orders(  # noqa:C901,WPS211,WPS213,WPS231
     db_session, make_address, make_courier, make_restaurant, make_order,
 ):
     """Sanity check the all the `make_*` fixtures.