Subscribe to Mailchimp `attendees_list' with TID merge tag.

Adds a new merge tag, for the ticket id, which will be used during
collection.
Change ticket price in Stripe and JS.

Following last week's ticket price change, this commit changes the price
both for Stripe, as well as for the dynamic price tag on the ticket
index page.
Subscribe user accounts to Mailchimp after validation.

Previously, it subscribed after update. This would cause the Mailchimp
list to not be synced with the account holder list (in the Rails
database), since some people's accounts haven't been updated (e.g. they
haven't logged in, for example). This change subscribes all accounts.