Currently we only check if some items are present in submitted JSON (see SANITY_CHECK_KEYS list at https://github.com/metabrainz/acousticbrainz-server/blob/master/db/data.py#L16-L30 and code that uses it). However it's easy to put any value into `lowlevel`, for example. There are two problems with this:
1. High-level extractor depends on values in keys defined there.
2. It's bad to accept arbitrary data (even if it's somewhat limited). See https://www.owasp.org/index.php/Don't_trust_user_input.

We know exactly what kind of output low-level extractor produces and submits to AcousticBrainz, so it shouldn't be a problem to make sure that structure and types are correct. We can use JSON Schema for this purpose.