The CMS uses deprecated synchronous XMLHttpRequests

[GuySartorelli]

When navigating to an edit form for a page, the following warning is displayed in the console:

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience. For more help http://xhr.spec.whatwg.org/

The spec says

Synchronous XMLHttpRequest outside of workers is in the process of being removed from the web platform as it has detrimental effects to the end user’s experience. (This is a long process that takes many years.) Developers must not pass false for the async argument when the current global object is a Window object. User agents are strongly encouraged to warn about such usage in developer tools and may experiment with throwing an "InvalidAccessError" DOMException when it occurs.

This isn't currently causing problems, but it definitely will eventually when browsers simply stop supporting that behaviour. This has been deprecated for a long time so the functionality could theoretically stop working at any moment.

It's possible this is also done in other places which is why I've opened this issue in admin instead of cms.

Explicit steps to reproduce:

1. navigate to /admin/pages
2. Click on any page in the site tree

This is not reproducible if you are already viewing an edit form - you must navigate to /admin/pages first.

Acceptance criteria

• The CMS does not use any deprecated synchronous XMLHttpRequest

[maxime-rainville]

It won't make the cut off for CMS5. Our assumption is that this can be ship in a minor later on.