Remove the managed_by_transition column from the sites table

- This is a part of retiring redirector.
Fix tests failing due to the removal of the database column

- Within this, remove most references to managed_by_transition,
  including SQL scopes.
Remove managed_by_transition from the hosts API

- In doing this, the hosts API no longer knows about
  managed_by_transition, and no longer refers to a site of a host being
  managed by transition. We're removing two hosts features that
  reference errors in the JSON, as there will be no errors now because
  it is just a list of hosts. Jenny and I discussed making the hosts API
  error when there is no data, but that would be inconsistent with the
  rest of the app's behaviour.
Don't import the data/sites directory from redirector anymore
Merge pull request #413 from alphagov/remove_managed_by_transition

Remove references to `managed_by_transition`