Hide the participants close button on larger screens

We need it for mobile where (eventually) we might have the participants
sidebar occupy the whole screen.

css/converse.css

@@ -11800,6 +11800,10 @@ body.converse-fullscreen {
 #conversejs.converse-mobile .chatroom .room-invite span .invited-contact {
   margin: 0 0 0.5em -1px; }
 
+@media (min-width: 768px) {
+  .occupants .occupants-header .hide-occupants {
+    display: none; } }
+
 #conversejs .chatbox.headlines .chat-head.chat-head-chatbox {
   background-color: var(--headline-head-color); }
 

sass/_chatrooms.scss

@@ -494,3 +494,13 @@
         }
     }
 }
+
+@include media-breakpoint-up(md) {
+    .occupants {
+        .occupants-header {
+            .hide-occupants {
+                display: none;
+            }
+        }
+    }
+}