WIP: enhance: replace variable id fetching with data-api

[Marigold]

Replace fetching by variable ID from owid website by fetching data directly from data-api. This eliminates the need for special cases & additional data and variable -> ID mappings. It runs against local data-api only and we're waiting for it to be deployed somewhere publicly.

Be aware that code that is currently in master doesn't work since variable endpoint has been split into /data and /metadata parts.