bug-1928808: fix _report_type validation

[willkg]

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.

I did manual testing:

1. go to http://localhost:8000/ and chose "TopCrashers" from the dropdown in the quick navigation bar
2. on the TopCrashers report, click through the three report types -- this should work fine
3. now manually edit the url and change _report_type to something like "pumpkin" -- this should return an HTTP 400 error page

[willkg]

Thank you!