Upgrade warthog to support concrete types in JSONField and update variants implementation

[dzhelezov]

Warthog now natively support concrete JSON types, which we use for variants: https://github.com/goldcaddy77/warthog/commit/5e137ced2a386509ac449e44562cd375699da6a4

After the upgrade, we should revisit the variant implementation, especially the hack around adding extra fields when the variant field is requested in the input query.