InstantAnswer.pm: Make the /ia/json/topic/ endpoint work.

This is a way for something like `/ia/json/topic/special%20interest` endpoint to work properly. What it does is that it goes through the whole resultset and filters out IAs that didn't match the value.

@MariagraziaAlastra @jbarrett This works, but there's a fair bit of code. I was wondering if you know if there's an easier way to do this in DBIx.
I don't think we need to re-evaluate the first two of these for every $ia