feat: remove deleted image from content

[hip3r]

Summary

Added configuration option for removing deleted image from content. Instead of saving empty string.
closes #7120
closes #7186

I wasn't sure that this should be working by default, that's why I extended cms configuration with new option remove_empty_image_field. conected PR for docs will follow.

Implementation is overlapping with logic for #3314 , which inserts default values for all removed entries. The same reason prevented me from saving image value as null or undefined instead of empty string.

Test plan

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AFTER:

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[demshy]

Instead of prop drilling the config, it might be more efficient to read the config once before calling serializeValues and add this option to the fields that are passed into the function. The state should be accessible on that point, I think. This approach would also maintain the possibility for adding this feature to specific fields in the future (or now?). What do you think?

We also have a chance to solve two problems in one go here and extend this to other field types (@martinjagodic I think this might interest you).

[demshy]

Do you think we could find a way to make this non-global?

[martinjagodic]

We also have a chance to solve two problems in one go here and extend this to other field types

We don't have to do this in this PR, but I would anticipate such a feature in config. So instead of remove_empty_image_field we could have remove_empty_fields, which would accept an array of widgets?