Add support for WebP image format

What does this MR do?

Adds support for WebP image format.

With Safari 14 supporting WebP on iOS as well as Mac, WebP will soon be available in all major browsers (pending users updating):

• https://caniuse.com/webp
• https://9to5mac.com/2020/06/24/apple-adds-webp-hdr-support-and-more-to-safari-with-ios-14-and-macos-big-sur/

This should make web / Flutter / Android developers really happy, of which some are already using WebP images in their projects to decrease app sizes and network load times.

Screenshots

(no screenshots yet)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

This MR does not touch any security-related stuff (like credentials, auth methods etc.).