Add a 'managed_by_transition' flag to Site, and use it for a hosts API

The flag reflects the intention to manage a site's mappings in Transition and have its requests sent to Bouncer by the CDN. Its initial use case is for the hosts API so that we can configure the CDN to send requests to Redirector or Bouncer on a per-site basis. That means that it is the switch we can flip to migrate sites from Redirector to Bouncer.