do not violate uniq email index in database

when scrubbing user info the email must be unique in the database index. This index is used for signups / finding by email etc do I didn't want to remove the uniq key. Instead the deleted email address now contains the user primary key id, e.g. "deleted_user_1@zooniverse.org".
Merge pull request #203 from camallen/error_delete

do not violate uniq email index in database