Make bibref query case-insensitive

See commit messages for more details.

This addresses tobie/specref#22
> AFAICT ref-list.json has been created automatically from inconsistent data, so that's a very weak argument.

The point of ref-list.json is to guarantee that no references are removed by accident when updating the db, so that something referenced today doesn't start erroring tomorrow.

> I don't want to spend more time discussing this. Just tell me whether it's still a blocker for you and I'll fix those few cases manually even though

Yes please. :)

> I'm still convinced that it is a completely senseless exercise.

It's not. It's a guarantee for the users of this db that we're not going to break their references with minor updates to the db. That was one of the requirements to spin specref out of Respec which we've continued to honor so far.