mosaico-wizard.html - "Continue" and "Submit" should use similar check
mosaico-wizard.html - "Continue" and "Submit" should use similar check

The "Continue" and "Submit Mailing" buttons serve a similar role (being the
finishing button within a wizard-step; both blocking on completion of their
form).  The "Submit Mailing" button was copy-pasted from a different
snippet, and its conditional wasn't correct in the new context.

With this revision, the "Submit" uses the same kind of check as "Continue".

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