Set managed_by_transition to false for newly-imported sites from Redirector

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.

(Sorry for whitespace changes here - it's a long flag name and the indentation would have been all wrong :( )

We don't have any tests at the moment for import behaviour when there are existing sites which could be updated/overwritten. I have tested this locally by setting the flag to true for a site, running import:all and verifying that the flag is not set back to false.