For some reason I thought this was already happening, but it isn't! We should make a new version of the common schema that sets additionalProperties: false. Specific schemas (like mediawiki/recentchange)can override this if they need to.

To do this we will make a new version of the common schema. I wouldn't bother updating the $refs to this new version until we need to actually make a change to existent schemas, but at least all new schemas (e.g. the upcoming eventlogging ones) will use the new common schema.