Disable popups on non-AMP pages with POST form elements

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go 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:

Because we load amp-form script on non-AMP pages to handle forms in AMP way, some other forms might be broken.
More context: ampproject/amphtml#27638

Closes #100

How to test the changes in this Pull Request:

1. Create a page with a form that has method="POST"
2. Create a popup that will appear on this page (e.g. sitewide overlay popup in test mode)
3. Visit page with AMP disabled (disable plugin or set mode to Transitional)
4. Observe that the popup does not show up
5. Observe that the form can be submitted without errors

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[jeffersonrabb]

Looks good. We'll come back to this when we know more about the AMP issue.

[matticbot]

🎉 This PR is included in version 1.3.1 🎉

The release is available on GitHub release

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[matticbot]

🎉 This PR is included in version 1.2.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀