Fix Content Editor QA test

What does this MR do and why?

It fixes the Content Editor’s QA test that focuses on ensuring that users can upload images using the Content Editor. The test fails because it doesn’t have a mechanism to detect when the image uploading process finishes. This Merge Request solves this problem by adding a data-uploaded attribute to image, audio, and video elements when the uploading process finishes.

The e2e and QA tests can use data-uploaded to detect the uploading state transition.

Screenshots or screen recordings

This MR doesn’t introduce user-facing changes.

How to set up and validate locally

Run the Content Editor QA test locally using the following command:

> cd qa && GITLAB_PASSWORD=<insert_password> bundle exec rspec qa/specs/features/browser_ui/3_create/wiki/content_editor_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related #373093 (closed)