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

migrations/versions/rev_20210102_18_888e352d7526_add_pixel_grid.py

@@ -36,7 +36,11 @@ def upgrade():
             onupdate='RESTRICT',
             ondelete='RESTRICT',
         ),
-        sa.UniqueConstraint('side_length', name=op.f('uq_grids_on_side_length')),
+        sa.UniqueConstraint(
+            'city_id', 'side_length', name=op.f('uq_grids_on_city_id_side_length'),
+        ),
+        # This `UniqueConstraint` is needed by the `addresses_pixels` table below.
+        sa.UniqueConstraint('id', 'city_id', name=op.f('uq_grids_on_id_city_id')),
         schema=config.CLEAN_SCHEMA,
     )
 
@@ -85,19 +89,13 @@ def upgrade():
         schema=config.CLEAN_SCHEMA,
     )
 
-    # These `UniqueConstraints`s are needed by the `addresses_pixels` table below.
+    # This `UniqueConstraint` is needed by the `addresses_pixels` table below.
     op.create_unique_constraint(
         'uq_addresses_on_id_city_id',
         'addresses',
         ['id', 'city_id'],
         schema=config.CLEAN_SCHEMA,
     )
-    op.create_unique_constraint(
-        'uq_grids_on_id_city_id',
-        'grids',
-        ['id', 'city_id'],
-        schema=config.CLEAN_SCHEMA,
-    )
 
     op.create_table(
         'addresses_pixels',