Discontinue removing sizes attribute when converting img to amp-img

[westonruter]

Feature description

In #4548 the validator was updated to introduce new disable-inline-width layout attribute. This will allow us to discontinue removing the sizes attribute on amp-img which the image sanitizer is currently doing:

amp-wp/includes/sanitizers/class-amp-img-sanitizer.php

Lines 333 to 334 in 014bbbb

	// Remove sizes attribute since it causes headaches in AMP and because AMP will generate it for us. See <https://github.com/ampproject/amphtml/issues/21371>.
	unset( $new_attributes['sizes'] );

This was confirmed in ampproject/amphtml#27083 (comment).

The sizes attribute was removed to fix long-standing issues with images:

• Images on attachment page appear stretched #1237
• <amp-img> sizes attribute behavior seems different from non-AMP <img> amphtml#17053
• Revisit aligned images with inline width larger than max getting displayed off-center #1086
• amp-img receives an unexpected inline style of width:100vw #1305
• Try: Experiment with amp-img-auto-sizes #1939
• …

It was removed as part of a PR to “definitively” fix the issues: #2036.

However, now that the disable-inline-width attribute is available to disable AMP's sizes behavior, we can add it automatically when converting img into amp-img (or amp-anim), and persist the sizes attribute as well.

This will involve re-checking the above issues for regressions!

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This should be done early in the 1.6 release to give time for bugs to shake out.

[amedina]

@schlessera Please add QA/Testing criteria for this issue and associated PRs.

[schlessera]

QA passed

The blocking attributes were correctly removed, the boilerplate was removed as a consequence of that, and none of the resulting images were distorted.

There was one bug that was already discovered here but hasn't been fixed yet. I split that issue out into a new bugfix PR (#5263) so that this one can pass QA.