Fix ApplicationUserModelID in the start menu shortcut

The application expected "com.squirrel.mattermost.Mattermost"
due to electron-winstaller. But probably the package name,
"mattermost-desktop" was used when creating the shortcut.

This might affect the internal name of .deb package.

Close #287

src/main.js

@@ -30,6 +30,7 @@ if (process.platform === 'win32') {
   }
 }
 
+app.setAppUserModelId('com.squirrel.mattermost.Mattermost'); // Use explicit AppUserModelID
 require('electron-squirrel-startup');
 
 const fs = require('fs');