[MM-55153] Consolidate Desktop App API, use new contextBridge endpoints when available (#25438)

* Create Desktop API module, migrate message passing code

* Changes to use new API

* Use Desktop API to notify when mentions/unreads/expired changes

* Expose Desktop App module to plugins

* Fix lint

* PR feedback

* Fixed an issue where I forgot to check if the method exists first

* Slight API changes

* Fix package

* Convert all to class, add comments, small reworks for PR feedback

webapp/channels/src/plugins/export.js

@@ -19,6 +19,7 @@ import Avatar from 'components/widgets/users/avatar';
 
 import {getHistory} from 'utils/browser_history';
 import {ModalIdentifiers} from 'utils/constants';
+import DesktopApp from 'utils/desktop_api';
 import messageHtmlToComponent from 'utils/message_html_to_component';
 import * as NotificationSounds from 'utils/notification_sounds';
 import {formatText} from 'utils/text_formatting';
@@ -103,3 +104,6 @@ window.ProductApi = {
     getRhsSelectedPostId: getSelectedPostId,
     getIsRhsOpen,
 };
+
+// Desktop App module containing the app info and a series of helpers to work with legacy code
+window.DesktopApp = DesktopApp;