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

* play around

* Flip things in presale

* Convert backend

* Remove test settings

* Safe getattr

src/pretix/base/context.py

@@ -4,7 +4,9 @@ from django.conf import settings
 
 
 def contextprocessor(request):
-    ctx = {}
+    ctx = {
+        'rtl': getattr(request, 'LANGUAGE_CODE', 'en') in settings.LANGUAGES_RTL,
+    }
     if settings.DEBUG and 'runserver' not in sys.argv:
         ctx['debug_warning'] = True
     elif 'runserver' in sys.argv: