Fix media image select

[myovchev]

Summary

Fixes a regression, where images in Media manager are not selectable (click on an image does nothing) in both default and relationship mode.

The source of the problem is a :key prop that is randomized on click, probably due to an immediate (outside) re-rendering of the component. The provided solution: memoize media item keys so that the "key" is attached to an image item just once.

What are the specific steps to test this change?

Selecting an image in Media Manager works as expected.

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

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