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:

#3783 introduced another optional module, but one that's not related to the Syndication section in our dashboard. Since #3783 expects the optional module settings to be available in the Newspack\Settings class, which has been deprecated with its methods moved to other classes throughout the wizard app, it's currently throwing a fatal.

This PR moves all methods and constants related to optional modules to a new Optional_Modules class for clarity.

How to test the changes in this Pull Request:

1. On epic/ia, observe a fatal error when loading any WP admin page.
2. Check out this branch and confirm the fatal no longer occurs.
3. Confirm that the settings in the Newspack > Settings > Syndication page display and update as expected.
4. Confirm that testing instructions in feat(woo-member-commenting): optional module for member commenting #3783 work as expected.

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?