Implement notifications for admin users (#700)

* First stab at notification settings

* Add "global" setting for notification levels

* Trigger notification task

* Get users with permission for event

* Actually send notification emails

* More notifications

* Allow to turn off notifications

* Link in email to pause all notifications

* Add NotificationType to wordlist

* Add notification tests

* Add documentation

* Rebase fixes

src/pretix/static/pretixcontrol/scss/_forms.scss

@@ -263,3 +263,11 @@ div.scrolling-multiple-choice, div.scrolling-choice {
     margin-top: 0;
   }
 }
+table td > .checkbox {
+  margin: 0;
+  position: static;
+}
+table td > .checkbox input[type="checkbox"] {
+  margin: 0;
+  position: static;
+}

src/pretix/static/pretixcontrol/scss/main.scss

@@ -121,6 +121,9 @@ h1 .btn-sm {
     padding-top: 20px;
 }
 
+.helper-display-block {
+    display: block !important;
+}
 .helper-display-inline {
     display: inline !important;
 }