Page builder check for shortcodes in admin

[derekashauer]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Resolves #3213

Check if a page builder is active in the admin before loading the preheader for shortcodes. Uses page builder's internal methods whenever possible, falls back to more simple URL param check for some.

Works with:

• Elementor
• Beaver Builder
• Visual Composer
• Divi
• Avada Fusion
• Bricks Builder

How to test the changes in this Pull Request:

1. Enable a page builder
2. Edit a page with the [pmpro_billing] (or any shortcode to fully test) in the page builder

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Check if a page builder is active in the admin before loading the preheader for shortcodes