Automatic update checks (#434)

* Basic update checks

* Fix issues pointed out by @rixx

* First test

* Add tests

* Even more tests

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

@@ -27,6 +27,10 @@ nav.navbar {
     display: inline;
 }
 
+nav.navbar .danger, nav.navbar .danger:hover, nav.navbar .danger:active {
+    background: $brand-danger !important;
+}
+
 .navbar-header .navbar-events {
     color: white;
     padding-top: 6px;