Fix a 500 when viewing payments in the admin panel.

"is not None" is not a valid Jinja2 test; "is not none" is, because "none" is
the name of a Jinja2 test.

templates/admin/payments/payment.html

@@ -16,7 +16,7 @@ <h2>Payment {{ payment.id }}</h2>
 {% endif %}
 {% if payment.provider == 'banktransfer' and payment.state == 'inprogress' %}
 <a class="btn btn-primary" href="{{ url_for('.reset_expiry', payment_id=payment.id) }}">Reset expiry</a>
-<a class="btn btn-warning {% if payment.reminder_sent_at is not None -%} disabled {%- endif %}" href="{{ url_for('.send_reminder', payment_id=payment.id) }}">Remind</a>
+<a class="btn btn-warning {% if payment.reminder_sent_at is not none -%} disabled {%- endif %}" href="{{ url_for('.send_reminder', payment_id=payment.id) }}">Remind</a>
 {% endif %}
 {% if payment.provider == 'stripe' %}
 <a class="btn btn-success" href="{{ url_for('.update_payment', payment_id=payment.id) }}">Update</a>