[BD-46] fix: show skeleton while loading src in CardImageCap

[khudym]

Description

issue with the fact that the markup jumps when loading a picture.
The CardImageCap component will display the Skeleton if Contexts isLoading=true (but this applies to the data itself), if isLoading=false, the img tag with the received data will be rendered, and the actual loading of the image will begin.

Couple solutions:

• show Skeleton not only when data isLoading, but when image is loading also; (here)
• prefetch images before render; PR(hook)

Issue#2480

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[openedx-webhooks]

Thanks for the pull request, @khudym!

When this pull request is ready, tag your edX technical lead.

[monteri]

Please, rename to pgn__image-loader

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