Add support for inviting form submission over email

[signebedi]

This can be an option in the form config dictionary that, when set to true, allows you to share the form with a user via email. It should inherit the unregistered_submission_method - that is, if it is email_validated_create_user, then we create the user and email them, if it is email_validated, then we simply send it. This could, in fact, just wrap the unregistered form submission process by enabling users to share forms via a UI click interface. Alternatively, we can make this a group permission. This would mean using probably the same route for admins as regular users... The share symbol would probably be: https://icons.getbootstrap.com/icons/share/ or https://icons.getbootstrap.com/icons/send/.

Originally posted by @signebedi in #357 (comment)