Set managed_by_transition to false for newly-imported sites

The flag already defaults to true because all new sites we add directly
to Transition should be managed there, but any new sites which are
added to Redirector first should have the flag set to false.
Test that we're leaving old sites alone
Merge pull request #69 from alphagov/set_managed_by_transition_on_import

Set managed_by_transition to false for newly-imported sites from Redirector