[expeditor] Modernize to use subscriptions
[expeditor] Modernize to use subscriptions

The artifact_actions keyword has been deprecated, port to the
subscriptions api instead.  This work leans heavily on this page
https://expeditor.chef.io/docs/getting-started/subscriptions/#artifact_actions,
but I'm not 100% sure I did everything right. In particular, I'm not
sure about how this will interact with there being two branches being
promoted, master and 12-18-stable.

Signed-off-by: Mark Anderson <f1b5a91d4d6ad523f2610114591c007e75d15084@chef.io>

chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server,chef/chef-server