Check event online registration settings compatibility with Unsupervised dedupe rule

If you have no email address in the profile(s) used for event registration, or if you allow multiple registrations with the same email address, you'll need a dedupe rule that has a chance of matching contacts without trying to use the email address.

This will check the situation and post a warning about it if there's no chance of matching duplicates with your settings.

This issue follows CRM-14134, which has the Unsupervised rule be used for all participants no matter whether the "Allow multiple registrations from the same email address" box is checked.