rsvp: another round of improvements

[foltik]

• event invite/confirmation/day-of emails now have the flyer in the header, and fixed date/time spacing
• event editor now shows both rsvped and in-cart count for each spot
• ability to duplicate and delete events
• ability for admins to manually add/remove attendees for an event
• better attendees page styling on mobile
• better parsing and error handling when importing email lists