Image Compare Block: add option to edit / replace existing images

> Once you've uploaded images, you cannot edit / replace one or both of them.

> This is known, initially this was considered an added layer of user interface complexity and that it would be easier to just delete the block and start over then add in a complex click sequence to select to replace and not interfere with the slider. However, we might be able to do something with the new Replace Image toolbar and G2 interfaces.

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From #15598.

The issue concerns the Image Compare Block and references the Replace Image toolbar, G2 interfaces, and issue #15598. Start by reviewing those references and the existing block behavior. Confirm the intended interaction before implementation; done means one or both uploaded images can be edited or replaced without recreating the block or disrupting the slider.

Written by the indexing model from the issue text.