Let newdle creator choose whether to send invitation emails or not

Usecase: You want to have a list of invited people, but not immediately send an email to them.

Drawback: Unless we add an option to manually trigger emails (probably with the ability to customize them), there would be no way to send emails with individual invitation codes, so unless people log in their answers would create a new participant instead of updating the invited one.

Comment from a user:

> I just used the Newdle tool for the first time. During the last step, the tool sent out the invitation mails without my consent. So the Newdle invitation arrived in the mailboxes of the invitees before I could send out the related invitation mail, explaining what this meeting is about. I had foreseen to insert the link to the Newdle (as one can do in the doodle) in that mail.

Start at the newdle creation flow's last step, where invitation emails are sent. Trace how invited people and individual invitation codes are handled, then define the creator's send-or-not choice and how invitations can be sent later; done means both paths preserve the intended participant identity.

Written by the indexing model from the issue text.