Member Content: Automatically add grve-container for Impeka Theme

[n7studios]

Summary

Building on #983, automatically adds the grve-container class to the Member Content container when the Impeka theme is active.

Testing

• testRestrictContentByProductWithImpekaTheme: Test that the grve-container is automatically added to the Member Content container when the Impeka Theme is active.
• ThemeDetectionTest: Added integration/unit tests to test the convertkit_is_theme_active helper method

Checklist

• 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)

[github-actions]

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