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

noxfile.py

@@ -470,7 +470,7 @@ def init_project(session):
 
 
 @nox.session(name='clean-pwd', python=PYTHON, venv_backend='none')
-def clean_pwd(session):  # noqa:WPS210,WPS231
+def clean_pwd(session):  # noqa:WPS231
     """Remove (almost) all glob patterns listed in .gitignore.
 
     The difference compared to `git clean -X` is that this task