Address issue where <ul> is converted to an <amp-carousel>

[kienstra]

Thanks to @christianc1 for the detailed report of this in #1528.

Steps To Reproduce

1. In AMP Settings > Template Mode, select 'Classic'
2. Create a post using Gutenberg
3. Add a Custom HTML block with:

<ul><li><a href="#">click me</a></li></ul>

4. Save and go to the AMP URL for the post
5. Expected: this markup shouldn't be changed on the AMP URL
6. Actual: this is wrapped in an <amp-carousel>, though it's not a gallery block:

Background

AMP_Gallery_Block_Sanitizer iterates through the <ul> tags that have child nodes.

As @christianc1 mentioned, if this is in Classic mode, the sanitizer will have the argument of array( 'carousel_required' => true ).

This causes $is_amp_carousel to be true, even for something like <ul><li></li></ul>.

Approach

Assuming that the AMP_Gallery_Block_Sanitizer is only intended for the Gallery block, restricting it to that block would probably make sense.

It looks like the Gallery block should always have the class of wp-block-gallery. Its stylesheet only uses that class, and the image selector begins with ul.wp-block-gallery.

So this restricts the Gallery block sanitizer to only ul.wp-block-gallery, and it simply exits if the class isn't present:

Closes #1528.

[kienstra]

Pending Unit Test Passing

If it's alright, I'll look at the failed unit test tomorrow. It passes locally.

[westonruter]

It looks like the failed test may be due to something new introduced in WordPress 5.0.

How does this PR relate to #1527?

[kienstra]

Hi @westonruter,
This PR takes a different approach from #1527, though we might still consider #1527.

I think there's something needed to ensure the AMP_Gallery_Block_Sanitizer only operates on Gallery blocks. Like how this PR checks that the <ul> has the class of wp-block-gallery.