Revise SupportedPlatforms migration

:fork_and_knife:

This takes into account the new `CookbookVersionPlatform` join model in Supermarket, and only imports `SupportedPlatforms` which have not yet been imported. It also makes the progress bar only visible when `ENV['SUPERMARKET_DEBUG']` is set, and removes top-level exception handling since that will be handled by Sidekiq in practice.