fix: onboarding layout overflow and responsiveness

[abaicus]

Summary

• Fixes issue with overflow in starter sites onboarding layout container;
• Uses available media-queries system instead of inlining new ones

Will affect visual aspect of the product

YES

Test instructions

• Test the onboarding on all devices. Layout should look ok;
• On desktop, the container should be limited to 1400px;
• There should be no horizontal scroll bar;

Check before Pull Request is ready:

• [ ] I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #428.

[pirate-bot]

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[rodica-andronache]

tested and it's working well 👍

[pirate-bot]

🎉 This PR is included in version 1.2.19 🎉

The release is available on GitHub release

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