Widget Blocks in Gutenberg 8.9 are not disabled for Reader themes

[westonruter]

Bug Description

In Gutenberg 8.9, the the Widget Blocks feature was enabled by default (WordPress/gutenberg#24843, WordPress/gutenberg#24087). The result is that now when a Reader theme is active, a Widget Blocks section appears even though widgets are supposed to be disabled in Reader mode:

Expected Behaviour

Gutenberg's Widget Blocks functionality should be disabled in Reader themes.

Steps to reproduce

1. Enable Reader mode and select a Reader theme other than Legacy.
2. Activate Gutenberg 8.9.0
3. Open the AMP Customizer.
4. See the Widget Blocks section.

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Acceptance criteria

• The Widget Blocks section should not be shown in the AMP Customizer.

Implementation brief

• We should perhaps empty out the $wp_registered_sidebars global in \AmpProject\AmpWP\ReaderThemeLoader::disable_widgets(), though this method may get called to early. We may need a new disable_sidebars() method that run the latest that register_sidebar() and register_sidebars() may be called.

QA testing instructions

Demo

Changelog entry

[westonruter]

Note to make tests pass in #5319, I had to remove_filter( 'customize_loaded_components', 'gutenberg_remove_widgets_panel' ) in a test: 521737d.

[westonruter]

There is also a filter and theme support which may be used. See https://make.wordpress.org/core/2020/09/03/whats-new-in-gutenberg-2-september/

[jwold]

QA passed.

I setup Local with with 8.9 Gutenberg plugin and latest DEV build of AMP.

I'm not seeing the Widget section in Customizer: https://d.pr/i/u3ckqb

[jwold]

QA passed passed. (Needed to run off the 2.0 branch)