[MM-50995] Harden Calls events handling (#2571)

* Simplify server switching logic in calls events

* Harden calls events handling

* Fix subpath

src/types/calls.ts

@@ -1,10 +1,8 @@
 // Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
 // See LICENSE.txt for license information.
 export type CallsWidgetWindowConfig = {
-    siteURL: string;
     callID: string;
     title: string;
-    serverName: string;
     channelURL: string;
 }
 
@@ -38,3 +36,5 @@ export type CallsErrorMessage = {
 export type CallsLinkClickMessage = {
     link: string | URL;
 }
+
+export type CallsEventHandler = ((viewName: string, msg: any) => void) | ((viewName: string, opts: Electron.SourcesOptions) => Promise<void>);