amp lightbox animate-in attribute removed by sanitizer

[p2er]

Description
Optional attribute animate-in is being removed from amp-lightbox and therefore does not allow animation configuration for lightbox even though included in the official documentation: https://www.ampproject.org/docs/reference/components/amp-lightbox#animate-in-(optional)

How to demo

• Add lightbox to page template
• Create page
• Open AMP version of page
• IS: animate-in attribute has been removed
• SHOULD: still have animate-in attribute in page MARKUP

Example

<button on="tap:demo-lb.open">open</button>
<amp-lightbox id="demo-lb" layout="nodisplay" scrollable animate-in="fly-in-top">
<button on="tap:demo-lb.close">close</button>
<p>This lightbox should animate into viewport from top.</p>
</amp-lightbox>

[hellofromtonya]

@p2er I can replicate this issue.

We'll need to update the 'attr_spec_list for amp-lightbox in AMP_Allowed_Tags_Generated

https://github.com/Automattic/amp-wp/blob/677b7617ffb47094dab04f76e91dafb57e67ad0c/includes/sanitizers/class-amp-allowed-tags-generated.php#L2264-L2286

While in there, we should also check the attributes to the spec for amp-image-lightbox.

[westonruter]

This will be resolved by #1315. We just need to finish it out.

[kienstra]

Moving To 'Ready For Merging'

If it's alright, I'm moving this to 'Ready For Merging,' as I don't think it needs functional testing. Feel free to move it back.