Image Compare Block: add uploading progress indicators

> As we discussed earlier, I find the lack of progress indicators disturbing. It's hard to know that the block is really working when you upload images.

> Agree, I think this can be a nice feature to add. Right now it matches the core "Media & Text" block, the current way to add an indicator is a bit complex. I plan to write up a ticket for improving media upload experience.

> Agreed — for V2. But we might even want to look at making that PR to the core project, if we can somehow benefit Media & Text, then perhaps we would not have to write one-off code for this block, and benefit both.

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From #15598.