[account_mtd_vat] Create log entry as early as possible in submission process

Immediately write submission log entry on button press at earliest technical opportunity.  Preferably immediately after guard clauses.

Set hmrc_request_state {'not_sent', 'sent', 'replied_success', 'replied_failure', 'timed_out'} on response from HMRC (timed-out iff we can detect that).

Set 'complete' flag at very end.

Remember to commit() at those key points.