S4 signup: only show public key field if the user wants us to use PGP.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
S4 signup: help text for Card number and CVC fields.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
S4 signup: move JS initialization from the HTML to subscription_signup.js,
and use jQuery's ready handler for portability to IE.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
S4 signup: simpler way of showing the form.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
payment_verified: break into paragraphs, say that support@ is the address for cancellations etc.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>