Search filter api again fix

Supercedes @tx2z's search filter api PR:  https://github.com/mozilla/kuma/pull/2399

@tx2z:  I fixed merge conflicts...I think.  Can you test and make sure your work is as how you remember it?

@stephaniehobson:  Can you look at the CSS here and let @tx2z know if we should make updates?


I added that line to expose what was going on with a more clear message; previously the test wasn't failing, it was errorring with `IndexError` on the next line, since the outbox was empty.

This test [does exist in master](https://github.com/mozilla/kuma/blob/master/kuma/wiki/tests/test_templates.py#L501), and passes there.