[Glitch] Use material symbol for responsive menu toggle

Port SCSS changes from 7519ef2fa1 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/styles/admin.scss

@@ -65,16 +65,16 @@ $content-width: 840px;
           background: $ui-base-color;
         }
 
-        .fa-times {
+        .material-close {
           display: none;
         }
 
         &.active {
-          .fa-times {
+          .material-close {
             display: block;
           }
 
-          .fa-bars {
+          .material-menu {
             display: none;
           }
         }