Modify Express Transfer functionality to insert redirection at the beginning of the account transfer

The current Express Transfer functionality has misleading documentation and does not function as it should. Currently, the express transfer redirection happens once the account has been completely transferred to the new server. This means that the site is still live on the origin server with database writes happening and email arriving. These changes are not propagated to the destination server leaving the two accounts in an inconsistent state.

The Express Transfer function should install the redirect at the beginning of the transfer process, so that the site is effectively taken offline during the transfer, ensuring a consistent state to the site once the transfer is completed. Obviously the .htaccess redirection will be copied across as part of the migration process so it will need to be undone on the destination server after the migration has completed.