Process variants synchronously on attachment to close the window between
image upload and variant availability, guaranteeing that we won't have
lazy variant processing attempts in GET requests.
Tradeoff is that we do variant processing in upload requests, which is
actually desirable. We're working with images that should take
milliseconds to resize given that we'll already have the file on hand.
References https://github.com/rails/rails/pull/51951
to avoid implicitly creating a variant during a GET to a read replica,
which will raise an exception and cause the image to fail to load.
This feels pretty brittle, to be honest, and we'll need to be careful
in the future about creating and using new variants; we will probably
need to backfill the new variant for existing images because we can't
guarantee that they will be created implicitly/lazily if the GET lands
on the replica.
ref: https://fizzy.37signals.com/5986089/collections/7/cards/942
Jorge Manrubia