Rename the unaudited table to the "exemptions" table

[bholley]

I've concluded that the current name is confusing, and that it's more self-explanatory to talk about "audits, imports, and exemptions".

Given that we haven't done a proper release yet and things are still somewhat use-at-your-own-risk, I don't think an automatic migration is essential. But @Gankra thinks it might be easy to do with a serde annotation — if so we might as well do that.

[Gankra]

This should in principle be trivially accomplished by https://serde.rs/field-attrs.html#alias but I would like to avoid poking this while #225 and #230 are in flight and touching so much code.