Added preventDefault to suppress browser-specific shortcuts (#2995)

webapp/components/channel_header.jsx

@@ -144,6 +144,7 @@ export default class ChannelHeader extends React.Component {
     }
     openRecentMentions(e) {
         if ((e.ctrlKey || e.metaKey) && e.shiftKey && e.keyCode === Constants.KeyCodes.M) {
+            e.preventDefault();
             this.searchMentions(e);
         }
     }

webapp/components/navbar_dropdown.jsx

@@ -71,6 +71,7 @@ export default class NavbarDropdown extends React.Component {
     }
     openAccountSettings(e) {
         if ((e.ctrlKey || e.metaKey) && e.shiftKey && e.keyCode === Constants.KeyCodes.A) {
+            e.preventDefault();
             this.setState({showUserSettingsModal: true});
         }
     }