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/db/addresses.py

@@ -52,7 +52,7 @@ class Address(meta.Base):
 
     # Relationships
     city = orm.relationship('City', back_populates='addresses')
-    restaurant = orm.relationship('Restaurant', back_populates='address', uselist=False)
+    restaurants = orm.relationship('Restaurant', back_populates='address')
     orders_picked_up = orm.relationship(
         'Order',
         back_populates='pickup_address',