Migrate data periodically from the Opscode Community Site

:fork_and_knife:

This adds the [chef-legacy](https://github.com/gofullstack/chef-legacy) gem to the Gemfile and adds a worker to periodically run the rake task it provides. The worker runs every 3 hours, which should hopefully give the first run enough time to complete before subsequent runs. On my machine, the initial import takes about 45 minutes, but I suspect performance -- especially the IO-heavy cookbook dependencies import -- will differ on an EC2 box.

A PR is forthcoming to the Supermarket cookbook to set the `ENV['COMMUNITY_SITE_DATABASE_URL']` variable from the app data bag.