Self-cancel endpoint now rejects paid registrations (public, series_pass,
or paid member tickets) with a 403 pointing to /policies/refunds. Free
and $0-member registrations still self-cancel as before. Adds the
refunds policy page referenced in the error message.
Introduces /community-guidelines and /policies/{privacy,terms,[slug]} pages,
swaps the signup/invite checkbox from agreedToTerms to agreedToGuidelines,
adds Member.agreement.acceptedAt, and stamps the field when a Helcim
customer is created.
