Refactor Features page script

[JakePT]

Description of the Change

This PR refactors the dashboard script and adjusts the markup to:

• Remove the jQuery dependency.
• Treat each feature as an individual <form> to take advantage of form APIs to simplify the script and markup, and improve keyboard accessibility.
• Use correct elements for accessible toggles and buttons with appropriate aria attributes.
• Remove vestigial unused code and dependencies.

It does not adjust the JavaScript for the ElasticPress > Settings page, so that has been moved into a separate file.

Benefits

Improves keyboard accessibility, by things such as allowing submitting the forms with the Enter key. It also simplifies the code so that it does not depend on custom data attributes and form handling.

Possible Drawbacks

Not necessarily a drawback, but the change results in a visual change for the Learn more and Collapse buttons:

Verification Process

Go to ElasticPress > Features and adjust settings of all features and verify that they are saving correctly. Also check ElasicPress > Settings and verify that it functions as expected.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes #2496

Changelog Entry

• Improves accessibility of the Features screen.

[Rahmon]

@JakePT could you resolve the conflicts so I can review this PR?

[JakePT]

@Rahmon Conflicts resolved.

[felipeelia]

@JakePT the linter is complaining about a few things in the ES code, do you mind giving a look into it? Thanks