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

setup.cfg

@@ -121,6 +121,8 @@ per-file-ignores =
     migrations/versions/*.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,
+        # Do not worry about SQL injection here.
+        S608,
         # File names of revisions are ok.
         WPS114,WPS118,
         # Revisions may have too many expressions.