FlexibleImageWidget should have a null option

[amickan]

Is your feature request related to a problem? Please describe.
For archive items and display sets, users need to be able to remove existing images from archive items and display sets.

Describe the solution you'd like
Similar to the SelectUploadWidget, the FlexibleImageWidget should have a ---- option.
When a user selects that option, the image CIV should be removed from the archive item / display set.

As discussed here, before removing the CIV, however, we might need to attach the CIV to a new archive item / display set so that the user keeps permissions to the image.

I wonder if the latter is really necessary. If a user deletes a case, they might also lose permission to the image, so why not accept that for removing an image from an item as well? Maybe I'm overseeing some of the rammifications here. If we do need to add a new case, that needs to be made very clear so users don't accidentally end up with additional cases without knowing.