Add option to replace existing image format completely with imageapi_optimize_webp_responsive

Problem/Motivation

Having the original image format as a fallback creates a lot of extra DOM and seems less necessary now that webp is supported by most browsers.

Steps to reproduce

Proposed resolution

Create option to remove the original file format when rendering the responsive image. I think there's probably a more elegant solution to handle the idea of image derivatives and fallbacks in general, but it would be nice to still have this support for sites using this module.

Remaining tasks

User interface changes

API changes

Data model changes