Support for right-to-left languages (#1438)

* play around

* Flip things in presale

* Convert backend

* Remove test settings

* Safe getattr

src/pretix/plugins/stripe/templates/pretixplugins/stripe/pending.html

@@ -9,7 +9,7 @@
     <p>{% blocktrans trimmed %}
         You need to confirm your payment. Please click the link below to do so or start a new payment.
     {% endblocktrans %}
-        <div class="text-right">
+        <div class="text-right flip">
             <a href="{% eventurl event "plugins:stripe:sca" order=order.code payment=payment.pk hash=payment_hash %}"
                     class="btn btn-primary">
                 {% trans "Confirm payment" %}