Sync ElasticPress to 3.6.5

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Description

Synchronize develop with the 3.6.5 tag of the upstream.

Checklist

Please make sure the items below have been covered before requesting a review:

• [ ] This change works and has been tested locally (or has an appropriate fallback).
• [ ] This change works and has been tested on a Go sandbox.
• [ ] This change has relevant unit tests (if applicable).
• [ ] This change has relevant documentation additions / updates (if applicable).
• [ ] This change has the fix PRed upstream (if applicable). If not applicable, it has the relevant "// VIP: reason for the discrepancy with upstream" comment in places where the code is discrepant.

Steps to Test

The Ci ideally should pass.

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Conflicts:

• includes/classes/Indexable/User/User.php: conflicts in @codingStandardsIgnoreXXX tags; replaced with the upstream implementation (phpcs:disable) [link]
• includes/classes/Indexable/Post/Post.php: [link]; please compare with the original
• includes/classes/Elasticsearch.php: conflicts in PHP docblocks; trivial to resolve [link]
• dist/js/comments-script.min.js: replaced with the upstream version
• dist/js/autosuggest-script.min.js: replaced with the upstream version
• CHANGELOG.md: trivial to resolve