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/replay/test_simulations.py

@@ -0,0 +1,17 @@
+"""Test the ORM's `ReplaySimulation` model."""
+
+
+class TestSpecialMethods:
+    """Test special methods in `ReplaySimulation`."""
+
+    def test_create_simulation(self, simulation):
+        """Test instantiation of a new `ReplaySimulation` object."""
+        assert simulation is not None
+
+    def test_text_representation(self, simulation):
+        """`ReplaySimulation` has a non-literal text representation."""
+        result = repr(simulation)
+
+        assert result.startswith('<ReplaySimulation(')
+        assert simulation.city.name in result
+        assert simulation.strategy in result