Automatic update checks (#434)

* Basic update checks

* Fix issues pointed out by @rixx

* First test

* Add tests

* Even more tests

src/pretix/control/context.py

@@ -3,6 +3,8 @@ import sys
 from django.conf import settings
 from django.core.urlresolvers import Resolver404, get_script_prefix, resolve
 
+from pretix.base.settings import GlobalSettingsObject
+
 from .signals import html_head, nav_event, nav_topbar
 from .utils.i18n import get_javascript_format, get_moment_locale
 
@@ -53,4 +55,13 @@ def contextprocessor(request):
     elif 'runserver' in sys.argv:
         ctx['development_warning'] = True
 
+    ctx['warning_update_available'] = False
+    ctx['warning_update_check_active'] = False
+    if request.user.is_superuser:
+        gs = GlobalSettingsObject()
+        if gs.settings.update_check_result_warning:
+            ctx['warning_update_available'] = True
+        if not gs.settings.update_check_ack and 'runserver' not in sys.argv:
+            ctx['warning_update_check_active'] = True
+
     return ctx