Allow users to upload WebP images

[maxime-rainville]

WebP images support is ubiquitous now. There's not much point left to block people from uploading them to the CMS.

Acceptance criteria

• [x] WebP images can be uploaded to the CMS assets section
• [x] WebP files are handle has images and can be added ho WYSIWYG
• [x] Clear steps are provided for developers who wish to disable this feature.

Pull request

• [x] https://github.com/silverstripe/silverstripe-assets/pull/476
• [x] https://github.com/silverstripe/silverstripe-framework/pull/10282
• [x] https://github.com/silverstripe/silverstripe-userhelp-content/pull/172

[dhensby]

Can this be closed now #476 is merged?

[maxime-rainville]

Still need to update the changelog/doc to explain how to disable the feature for folks who don't want it.

[GuySartorelli]

All connected PRs are merged.