Made emoji size in scale with text size in channel header (#23743)

webapp/channels/src/sass/layout/_headers.scss

@@ -402,6 +402,15 @@
                     margin: 0;
                 }
             }
+
+            .emoticon {
+                width: 18.8px;
+                min-width: 18.8px;
+                max-width: 18.8px;
+                height: 18.8px;
+                min-height: 18.8px;
+                max-height: 18.8px;
+            }
         }
 
         .markdown__heading {