Fix regression in payment prompt

I didn't update the payment_method_token to stripe_customer_id in the
template where we prompt the user to back their tip with a credit card
if they haven't done so yet. #58

templates/participant.html

@@ -19,7 +19,7 @@ <h2>Your weekly {{ tip_or_pledge }} to {{ username }} is:</h2>
  {% end %}
  </div>
 
- {% if user.payment_method_token is None %}
+ {% if user.stripe_customer_id is None %}
  <p id="payment-prompt"{% if my_tip > 0 %} class="needed"{% end %}>
  <a href="/credit-card.html" class="highlight">Back your tip with a 
  credit card</a> to make sure your tip goes through!</p>