Add OrderHistory.first/last_order() methods

- get the `datetime` of the first or last order within a pixel
- unify some fixtures in `tests.forecasts.timify.conftest`

tests/db/fake_data/static_fixtures.py

@@ -67,4 +67,4 @@ def grid(city):
 @pytest.fixture
 def pixel(grid):
     """The `Pixel` in the lower-left corner of the `grid`."""
-    return db.Pixel(grid=grid, n_x=0, n_y=0)
+    return db.Pixel(id=1, grid=grid, n_x=0, n_y=0)