Show warnings about development mode and DEBUG mode

src/pretix/control/context.py

@@ -1,3 +1,4 @@
+import sys
 from django.conf import settings
 from django.core.urlresolvers import Resolver404, get_script_prefix, resolve
 
@@ -46,4 +47,9 @@ def contextprocessor(request):
     ctx['js_date_format'] = get_javascript_format('DATE_INPUT_FORMATS')
     ctx['js_locale'] = get_moment_locale()
 
+    if settings.DEBUG and 'runserver' not in sys.argv:
+        ctx['debug_warning'] = True
+    elif 'runserver' in sys.argv:
+        ctx['development_warning'] = True
+
     return ctx