Back-office registration of guest participants

Motivation: Currently it is not possible to add 'additional participants' (guests) to an event booking in the back end. Instead, you need to create a new booking for the guest record, and then there is no way to 'link' them to the lead booker.

Solution: Allow additional participants (guests) to be added to someone's booking, when managing participants through the back end, eg:

Previous work: See https://issues.civicrm.org/jira/browse/CRM-19047 . The PR here (https://github.com/civicrm/civicrm-core/pull/8676) works for adding guests to peoples' bookings in the back end for free events. But we ran into some problems with paid events, because of the way the contribution row is handled.

We are currently working on this and hope to submit a new PR that will work for free and paid events by the end of January.