Notifications: load WP.com tracking pixel with crossorigin attribute

#### Steps to reproduce the issue

For the [Web Stories WordPress plugin](https://wordpress.org/plugins/web-stories/) we set [cross-origin isolation](https://web.dev/coop-coep/) on some admin pages, which requires loading 3P resources with the `crossorigin` attribute.

If not done, loading these resources will fail with console errors. This is the case for the Notifications module. And potentially others as well.

1. Activate [Web Stories for WordPress](https://wordpress.org/plugins/web-stories/)
2. Activate Notifications module
3. Go to Stories -> My Stories or Stories -> Add New
4. Check console errors

#### What I expected

The tracking pixel should be loading just fine, no console errors.

For this, the `` needs to be loaded with the `crossorigin` attribute.

The image itself should have a `Access-Control-Allow-Origin: *` response header.

#### What happened instead

See console error because tracking pixel is not loaded with `crossorigin` attribute

**Screenshots**
If applicable, add screenshots to help explain your problem.

Start in the Notifications module and locate the WP.com tracking-pixel used on the affected admin pages. Reproduce with Web Stories enabled, inspect the browser console, and confirm the image request uses the crossorigin attribute and succeeds with the required CORS response header.

Written by the indexing model from the issue text.