Use React-based component for Stylesheets box on Validated URL screen

[delawski]

Summary

This PR replaces the server-side rendered "Stylesheet" meta box on the Validated URL screen with a React-based component fed via the REST API.

Tasks list (subject to change):

• [x] Create a new REST endpoint, GET /wp-json/amp/v1/validated-urls/:id/, that returns the validation data stored in a amp_validated_url post type. See #5552. The response data structure consists of the following fields:
  • id
  • url
  • date
  • author
  • stylesheets
  • environment
• [x] Ensure the REST endpoint is secure so that only authorized users are receiving the validation data.
• [x] Introduce an abstraction layer for the amp_validated_url custom post type in the ValidatedUrlDataProvider class.
• [x] Update the REST endpoint schema.
• [x] Implement a new React component that will look similar to the existing "Stylesheets" meta box:
  • [x] Add stylesheets summary table
  • [x] Add stylesheets details table
  • [x] Add CSS code diff component
  • [x] Add Sources component
• [x] Remove the legacy "Stylesheets" meta box along with code dependencies.
• [x] Preload validated URL, themes, and plugins data.
• [ ] Incorporate any applicable usability improvements from #5191.

Fixes #6441

Checklist

• [ ] My code is tested and passes existing tests.
• [x] My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[codecov[bot]]

Codecov Report

Merging #6442 (b9de4d5) into develop (24923a4) will increase coverage by 0.51%. The diff coverage is 56.80%.

:exclamation: Current head b9de4d5 differs from pull request most recent head 26525f8. Consider uploading reports for the commit 26525f8 to get more accurate results

@@              Coverage Diff              @@
##             develop    #6442      +/-   ##
=============================================
+ Coverage      75.52%   76.04%   +0.51%     
- Complexity      5987     5999      +12     
=============================================
  Files            190      266      +76     
  Lines          18037    19242    +1205     
=============================================
+ Hits           13623    14633    +1010     
- Misses          4414     4609     +195     

Flag	Coverage Δ
javascript	71.99% <58.88%> (?)
php	76.30% <53.62%> (+0.78%)	:arrow_up:
unit	76.30% <53.62%> (+0.78%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...validation/components/amp-document-status/index.js	100.00% <ø> (ø)
...nents/amp-validation-status/status-notification.js	100.00% <ø> (ø)
src/AmpWpPlugin.php	100.00% <ø> (ø)
src/Validation/URLValidationRESTController.php	86.13% <0.00%> (-6.42%)	:arrow_down:
...s/src/components/plugins-context-provider/index.js	7.40% <7.40%> (ø)
...ts/src/components/themes-context-provider/index.js	7.40% <7.40%> (ø)
assets/src/components/source-details/title.js	11.53% <11.53%> (ø)
assets/src/components/source-details/value.js	11.53% <11.53%> (ø)
...components/validated-url-context-provider/index.js	14.28% <14.28%> (ø)
src/Validation/ValidatedUrlData.php	18.89% <18.89%> (ø)
... and 103 more

[github-actions[bot]]

Plugin builds for 26525f8c91fedaaee4833d89c07f6cc210f2dc06 are ready :bellhop_bell:!

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• Download production build

[delawski]

The PR is now rebased onto develop and the conflicts have been resolved. Also, I've removed some bugs related to the Sources component.