Add templates for successful and cancelled payments

myproject/myapp/templates/payment_cancelled.html

+{% extends "_base.html" %}{% block content %}
+<h1>Payment Cancelled</h1>
+<p>Your payment has been cancelled.</p>
+<button><a href="{% url 'pricing' %}">Return</a></button>
+{% endblock content%}

myproject/myapp/templates/payment_success.html

+{% extends "_base.html" %}{% block content %}
+<h1>Success!</h1>
+<p>Your payment was successful and you have been credited with 1 token.</p>
+<button><a href="{% url 'users' %}">Return</a></button>
+{% endblock content%}