bug-1928808: fix _report_type validation

This fixes _report_type validation in the TopCrashers form so it doesn't
create bad Super Search queries which kick up an HTTP 500.

Previously, the form did no useful validation (why even have a form?).
Now it enforces that _report_type is one of "any", "hang", or "crash"
which are the only three values it should have. If it gets a different
value, the form will cause the view to raise an HTTP 400 with an error
saying that the _report_type value isn't one of the available choices.

webapp/crashstats/topcrashers/forms.py

@@ -16,4 +16,10 @@ class TopCrashersForm(BaseForm):
     _facets_size = forms.IntegerField(required=False)
     _tcbs_mode = forms.CharField(required=False)
     _range_type = forms.CharField(required=False)
-    _report_type = forms.CharField(required=False)
+
+    # NOTE(willkg): _report_type choices must match the TopCrashers form options in the
+    # jinja2 template
+    _report_type = forms.ChoiceField(
+        choices=[("any", "any"), ("hang", "hang"), ("crash", "crash")],
+        required=False,
+    )