Add auditable superuser mode (#824)

* Remove is_superuser everywhere

* Session handling

* List of sessions, relative timeout

* Absolute timeout

* Optionally pseudo-force audit comments

* Fix failing tests

* Add tests

* Add docs

* Rebsae migration

* Typos

* Fix tests

src/pretix/control/context.py

@@ -3,8 +3,10 @@ from importlib import import_module
 
 from django.conf import settings
 from django.core.urlresolvers import Resolver404, get_script_prefix, resolve
+from django.db.models import Q
 from django.utils.translation import get_language
 
+from pretix.base.models.auth import StaffSession
 from pretix.base.settings import GlobalSettingsObject
 
 from ..helpers.i18n import get_javascript_format, get_moment_locale
@@ -93,10 +95,19 @@ def contextprocessor(request):
     ctx['warning_update_check_active'] = False
     gs = GlobalSettingsObject()
     ctx['global_settings'] = gs.settings
-    if request.user.is_superuser:
+    if request.user.is_staff:
         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
 
+    if request.user.is_authenticated:
+        ctx['staff_session'] = request.user.has_active_staff_session(request.session.session_key)
+        ctx['staff_need_to_explain'] = (
+            StaffSession.objects.filter(user=request.user, date_end__isnull=False).filter(
+                Q(comment__isnull=True) | Q(comment="")
+            )
+            if request.user.is_staff and settings.PRETIX_ADMIN_AUDIT_COMMENTS else StaffSession.objects.none()
+        )
+
     return ctx