Event header: Fix wrapping of date in h2 header (Z#23245589)

This was already correct if the header was a h1

src/pretix/static/pretixpresale/scss/_theme.scss

@@ -75,6 +75,9 @@ h3, .h3 {
 
 h2.content-header {
   margin-top: 0;
+  small {
+    white-space: nowrap;
+  }
 }
 
 @media (max-width: $screen-xs-max) {
@@ -85,6 +88,7 @@ h2.content-header {
   }
   h2.content-header small {
     display: block;
+    white-space: normal;
   }
 }
 @media (min-width: $screen-sm-min) {