add a mechanism to reenable migration for edited card

Idea

 

Sometimes people (including often us here on wagn.org) edit a Cards on New Wagns and then end up setting it back to the original, or in any case wish that it would start being migrated again when it's updated on en.

 

(Currently we don't migrate a card that has been edited by anyone other than Wagn Bot.)

 

 

 

+relevant user stories