Fix imageLoading bug when scrolling back to design

What does this MR do?

Fixes a bug where if the user scrolled past an image such that it was no longer in view, and then scrolled back so that the image was again in view, a loading spinner would incorrectly be shown. This regression was introduced in !27460 (merged)

This MR adds code to show the loading spinner only if the image has never appeared in view.

It also adds and reorganises the tests in item_spec.js to cover this path.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team