Added a custom csrf failure view

src/pretix/base/templates/csrffail.html

@@ -0,0 +1,24 @@
+{% extends "error.html" %}
+{% load i18n %}
+{% block title %}{% trans "Verification failed" %}{% endblock %}
+{% block content %}
+    <i class="fa fa-frown-o big-icon"></i>
+    <h1>{% trans "Verification failed" %}</h1>
+    <p>{% blocktrans trimmed %}
+        We could not verify that this request really was sent from you. For security reasons, we therefore cannot process it.
+    {% endblocktrans %}</p>
+    {% if no_referer %}
+        <p>{{ no_referer1 }}</p>
+        <p>{{ no_referer2 }}</p>
+    {% elif no_cookie %}
+        <p>{{ no_cookie1 }}</p>
+        <p>{{ no_cookie2 }}</p>
+    {% else %}
+        <p>{% blocktrans trimmed %}
+            Please go back to the last page, refresh this page and then try again. If the problem persists, please get in touch with us.
+        {% endblocktrans %}</p>
+    {% endif %}
+    <p>
+        <a href="javascript:history.back()">{% trans "Take a step back" %}</a>
+    </p>
+{% endblock %}