Add toggle to manage whether native img is used instead of amp-img and add admin pointer

[westonruter]

Feature Description

With #6805 we're now defaulting to native img instead of amp-img. However, this could have unintended side effects (e.g #7029 (comment)) since amp-img has been around from the beginning and there may be CSS selectors written that specifically target amp-img. Previously the CSS sanitizer would convert img selector to amp-img (and amp-anim) while converting the elements in the DOM. But when using native image, no such conversion is done. So if someone was expecting to style an image with amp-img this would no longer match.

Therefore, in order to mitigate this major change it is warranted to add a new toggle to the Settings screen in the Advanced section for whether native img is enabled. True this can be managed via a PHP filter as well, but this would require a user to add a mini plugin which is greater friction than just a simple toggle. The toggle should be omitted or disabled if a site has any amp_native_img_used filter.

Upon upgrading to 2.3, there should be an admin pointer that alerts users to the fact that native images are now being used, with a deep link to that toggle in the Settings screen. The toggle description should advise users to check their site for issues with images rendering, and if so, to try turning off native images to see if that resolves the issue.

Acceptance Criteria

No response

Implementation Brief

No response

QA Testing Instructions

No response

Demo

No response

Changelog Entry

No response

[westonruter]

To avoid headaches with regressions where img isn't yet supported fully (e.g. #7029 (comment)), I think we should continue to default to amp-img and then have a toggle to opt-in to native img.

[pooja-muchandikar]

QA Passed ✅

• Toggle is implemented to manage whether to use native img instead of amp-img
• With this toggle being disable, the image will have amp-img by default
• If toggle is enabled, then image will use img ✅

Screen.Recording.2022-06-06.at.4.40.17.PM.mov

---