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