A smoother path from joining to buying a ticket

When a ticket needs a membership, buyers get sent to the right membership page and can come back and finish paying.

If a ticket is members-only, someone who has not joined yet now gets pointed at the correct membership page instead of a dead end. Once they have joined they can come back to checkout and finish the job.

The earlier part of event checkout got tidied up while we were in there, with clearer sold-out and sales-ended states and better spacing around event titles and dates.