Re-factor the ORM tests to use randomized fake data

- create `*Factory` classes with fakerboy and faker that generate
  randomized instances of the ORM models
- add new pytest marker: "db" are the integration tests involving the
  database whereas "e2e" will be all other integration tests
- streamline the docstrings in the ORM models

tests/db/fake_data/static_fixtures.py

@@ -0,0 +1,58 @@
+"""Fake data for testing the ORM layer."""
+
+import pytest
+
+from urban_meal_delivery import db
+
+
+@pytest.fixture
+def city_data():
+    """The data for the one and only `City` object as a `dict`."""
+    return {
+        'id': 1,
+        'name': 'Paris',
+        'kml': "<?xml version='1.0' encoding='UTF-8'?> ...",
+        '_center_latitude': 48.856614,
+        '_center_longitude': 2.3522219,
+        '_northeast_latitude': 48.9021449,
+        '_northeast_longitude': 2.4699208,
+        '_southwest_latitude': 48.815573,
+        '_southwest_longitude': 2.225193,
+        'initial_zoom': 12,
+    }
+
+
+@pytest.fixture
+def city(city_data):
+    """The one and only `City` object."""
+    return db.City(**city_data)
+
+
+@pytest.fixture
+def address(make_address):
+    """An `Address` object in the `city`."""
+    return make_address()
+
+
+@pytest.fixture
+def courier(make_courier):
+    """A `Courier` object."""
+    return make_courier()
+
+
+@pytest.fixture
+def customer(make_customer):
+    """A `Customer` object."""
+    return make_customer()
+
+
+@pytest.fixture
+def restaurant(address, make_restaurant):
+    """A `Restaurant` object located at the `address`."""
+    return make_restaurant(address=address)
+
+
+@pytest.fixture
+def order(make_order, restaurant):
+    """An `Order` object for the `restaurant`."""
+    return make_order(restaurant=restaurant)