Prevent profile form page from being skipped for new users

If a new user is being signed up for an event, after the email addresses are entered the buyer gets taken to the profile page. If, at that point, the buyer were to abort and later return to their cart and click "Check Out," they'll get taken to the payment page without ever being given the chance to fill out the profiles for any new users. (And required profile fields will throw errors confusingly.)

This patch will send the buyer back to the profile page, until it is submitted and profile values exist for each new user.

Note, this is slightly different than #728962: Force entry of empty required profile fields for existing users in that I believe this is a bug, and that issue is a feature request to send the buyer back to the profile page even for existing users if they have profile required fields that are not filled in.