Add IE11 banner to frontend (#4207)

src/pretix/static/pretixpresale/scss/main.scss

@@ -519,6 +519,13 @@ h2 .label {
     .progress-bar-#{$i} { width: 1% * $i; }
 }
 
+
+.old-browser-warning {
+    background-color: #ffe761;
+    padding: 32px; /* 30px + 2px optical compensation */
+    font-size: 30px;
+}
+
 @import "_iframe.scss";
 @import "_a11y.scss";
 @import "_print.scss";