Add AMP_Script_Sanitizer to replace noscript elements with their contents

[westonruter]

As noted in Implementing Interactivity:

the baseline experience after enabling AMP on an existing theme is that it largely behaves as if JavaScript is disabled in the user's browser. Many themes and plugins already have fallbacks in case JavaScript is not available so that will serve them well in AMP. Therefore, most of the work entailed is to progressively enhance a theme for AMP to restore features that were lost during this graceful no-JS degradation to AMP

Nevertheless, this is not totally true at present: the plugin's postprocessor does not currently do any transformation of noscript elements meaning that the no-JS fallbacks will be not be successfully used in AMP pages.

So this PR ensures that that a non-AMP document like:

<script>document.write('Scripts!')</script><noscript><em>No scripts!</em></noscript>

Will get served in AMP as:

<!--noscript--><em>No scripts!</em><!--/noscript-->

Thus it encourages the use of no-JS fallbacks for the baseline AMP experience as well as for visitors who have JS turned off in their browsers.

This will be further built upon in #1213 and #1032 with more intelligence about whether to promote the noscript element's contents or rather to convert a companion script to an AMP equivalent.

The base of this PR comes from #1127

[kienstra]

Approved

Hi @westonruter and @amedina,
This PR looks really good, and has very thorough PHPUnit tests. As you mentioned, this should help the user experience when scripts are removed.

When you have a chance, it'd be good to address the Travis build issues. I could help with that if you'd like.

[kienstra]

It's nice how this outputs the comment <!--noscript-->. This should help with debugging if needed, like on WordPress.org support topics.

[kienstra]

Nice thorough testing of the boilerplate not being removed.

[westonruter]

@kienstra thank you. I'm very confused about why the tests are failing. They all pass for me locally.

[kienstra]

@westonruter, that's strange, the tests pass locally for me also.

[westonruter]

Force-pushed a tweak. Maybe there was a caching problem. The test is failing as if these lines are not present:

--- a/tests/test-amp-script-sanitizer.php
+++ b/tests/test-amp-script-sanitizer.php
@@ -38,8 +38,6 @@ class AMP_Script_Sanitizer_Test extends WP_UnitTestCase {
 	 */
 	public function test_noscript_promotion( $source, $expected = null ) {
 		$dom       = AMP_DOM_Utils::get_dom_from_content( $source );
-		$sanitizer = new AMP_Script_Sanitizer( $dom );
-		$sanitizer->sanitize();
 		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom );
 		$whitelist_sanitizer->sanitize();
 		$content = AMP_DOM_Utils::get_content_from_dom( $dom );

[westonruter]

I'm pretty sure the problem is due to an old version of libxml and this bit of logic:

https://github.com/Automattic/amp-wp/blob/70711f0cd647f233b02ffa1bbbd58496eeeea510/includes/utils/class-amp-dom-utils.php#L77-L110