Fix a bug from trying to scrape all trips

I found a scraper (hello, Scrapy user!) trying to scrape all trips from
https://mitoc-trips.mit.edu/trips/?after=0001-10-17

The server crashed because I mistakenly assumed you can subtract 365
days from any given year. Let's not do that.

Separately, with *thousands* of trips, this view is getting pretty slow.
To monitor (and perhaps limit) scrapers, I might try to make the view a
bit more locked down. I don't *want* to paginate, but I don't want abuse
either.

ws/tests/views/test_trips.py

@@ -126,6 +126,17 @@ def test_trips_with_filter(self):
         self._expect_past_trips(response, [one_week_ago.pk, one_month_ago.pk])
         self._expect_link_for_date(soup, "2016-11-15")
 
+    def test_trips_with_very_early_date(self):
+        """You can ask for trips starting after the year 1."""
+        trip = factories.TripFactory.create(trip_date="2016-12-23")
+
+        # Filter based on a date in the past
+        response, soup = self._get("/trips/?after=0001-10-17")
+        self.assertFalse(response.context["date_invalid"])
+
+        self._expect_title(soup, "Trips after 1900-01-01")
+        self._expect_past_trips(response, [trip.pk])
+
     def test_upcoming_trips_can_be_filtered(self):
         """If supplying an 'after' date in the future, that still permits filtering!"""
         _next_week = factories.TripFactory.create(trip_date="2019-02-22")

ws/views/trips.py

@@ -487,7 +487,7 @@ def get_queryset(self):
         trips = super().get_queryset()
         return annotated_for_trip_list(trips)
 
-    def _optionally_filter_from_args(self):
+    def _optionally_filter_from_args(self) -> tuple[date, bool]:
         """Return the date at which we want to omit previous trips, plus validity boolean.
 
         If the user passes an invalid date (for example, because they were
@@ -505,7 +505,13 @@ def _optionally_filter_from_args(self):
             else:
                 start_date_invalid = False
 
-        return (start_date, start_date_invalid)
+        return (
+            # Guard against a weird edge case -- we can accept the year 1!
+            # We'll try to *subtract* time, and end up an exception rather than BCE datetimes
+            # There were no trips in our database before the 2010s anyway.
+            start_date and max(start_date, date(1900, 1, 1)),
+            start_date_invalid,
+        )
 
     def get_context_data(self, **kwargs):
         """Sort trips into past and present trips."""