Add ORM models for the simulation data

- add new ORM models `ReplaySimulation` and `ReplayedOrder`
  to store the data generated by the routing simulations
- add migrations script to create the corresponding database tables
  + create "replay_simulations" and "replayed_orders" tables
  + add missing check constraints to "orders" table
  + add unique constraint to "orders" table to enable compound key
  + drop unnecessary check constraints from the "orders" table
- add tests for the new ORM models
  + add `simulation`, `replayed_order`, `make_replay_order()`, and
    `pre_order` fixtures
  + add `ReplayedOrderFactor` faker class
- fix some typos

tests/db/fake_data/static_fixtures.py

@@ -54,10 +54,22 @@ def restaurant(address, make_restaurant):
 
 @pytest.fixture
 def order(make_order, restaurant):
-    """An `Order` object for the `restaurant`."""
+    """An ad-hoc `Order` object for the `restaurant`."""
     return make_order(restaurant=restaurant)
 
 
+@pytest.fixture
+def pre_order(make_order, restaurant):
+    """A scheduled `Order` object for the `restaurant`."""
+    return make_order(restaurant=restaurant, scheduled=True)
+
+
+@pytest.fixture
+def replayed_order(make_replay_order, order):
+    """A `ReplayedOrder` object for the `restaurant`."""
+    return make_replay_order(order=order)
+
+
 @pytest.fixture
 def grid(city):
     """A `Grid` with a pixel area of 1 square kilometer."""
@@ -68,3 +80,20 @@ def grid(city):
 def pixel(grid):
     """The `Pixel` in the lower-left corner of the `grid`."""
     return db.Pixel(id=1, grid=grid, n_x=0, n_y=0)
+
+
+@pytest.fixture
+def simulation_data(city):
+    """The data for the one and only `ReplaySimulation` object as a `dict`."""
+    return {
+        'id': 1,
+        'city': city,
+        'strategy': 'best_possible_routing',
+        'run': 0,
+    }
+
+
+@pytest.fixture
+def simulation(simulation_data):
+    """The one and only `ReplaySimulation` object."""
+    return db.ReplaySimulation(**simulation_data)