[participants] delayed onboarding emails

Support delaying onboarding. Invited as an attendee - staff can see they're there. Can remove them, etc. Series HQ clicks a button and that sends out invitation emails.

Start by tracing the participant onboarding and invitation-email flow, then find where staff can view and remove attendees and where Series HQ actions are handled. Confirm how delayed invitations should be triggered and what staff should see before implementing; done means staff can defer onboarding emails and Series HQ can send them later with the button.

Written by the indexing model from the issue text.