[FIX] Import transfer: movements & transfer with the same created_at

[franpb14]

Changes

• now when te movements are created they use the same created at as the transfer

Note: If we didn't want to touch the rest of the app and only make this change in the import, we could use transfer.movements.update_all(created_at: transfer.created_at) in process_row after the transfer save. However, I believe it makes more sense as I placed it, to ensure consistency between transfer and movements at all times

[sseerrggii]

Tested on staging ✔️
Works fine, thanks @franpb14