PLT-3473 Made shortcuts only work for CMD on OSX (#3479)

* checked for CMD key

* changed method of checking

webapp/components/channel_header.jsx

@@ -155,7 +155,7 @@ export default class ChannelHeader extends React.Component {
     }
 
     openRecentMentions(e) {
-        if ((e.ctrlKey || e.metaKey) && e.shiftKey && e.keyCode === Constants.KeyCodes.M) {
+        if (Utils.cmdOrCtrlPressed(e) && e.shiftKey && e.keyCode === Constants.KeyCodes.M) {
             e.preventDefault();
             this.searchMentions(e);
         }