Handle date ranges in SK responses better

[leonidasmi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Fixes an unreleased bug with the total Organic Session number that appear in the relevant widget.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• For getting the complete data about the last 28 days and the complete data of the 28 days before that, do a GET request to /wp-json/yoast/v1/time_based_seo_metrics?options[widget]=organicSessionsCompare
• Confirm that the response you get is like:

[
    {
        "current": {
            "sessions": 9
        },
        "previous": {
            "sessions": 12
        }
    }
]

• Go to the actual Google Analytics platform and search for organic sessions of the last 28 days compared with the organic sessions of the 28 days before that and confirm you get the same result
  • Specifically, you can do that by changing the date range at the top right corner of the Acquisition page in analytics and then enable Compare and select Preceding period:
    
• You can check now the sessions from the last 28 days compared with the 28 days before that:
  
• Confirm that the current sessions in our REST response is the same with the number of the period of the last 28 days in Analytics. And that the previous sessions in our REST response is the same with the number of the period of the last 28 days before the last 28 days.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes https://github.com/Yoast/reserved-tasks/issues/508

[thijsoo]

LGTM 👍