[account_mtd_vat] Re-check obligation on click of submit button

We've mostly solved this as part of T15219, but we're not currently syncing.  Instead we are flagging when we think it's done and checking before doing submission, for now.  Our hand was forced because, at least in the sandbox, we're not getting returned updated status from the obligations endpoint after submission.

This bug would now add a 'synchronise' step beforehand.