feat(reader-activation): customizable UI text for auth modal

[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:

Adds customizable text to the auth UI, which can serve as a place for the publisher to specify additional details about the authorization.

Engagement wizard	front-end (one of 3)

To Do

• labels in the Engagement wizard
• polishing the UI in the Engagement wizard

How to test the changes in this Pull Request:

1. View the auth modal UI, confirm nothing has changed
2. Open the Engagement wizard and observe a new section in the Advanced Settings: "Sign-in UI text"
3. Fill in the UI text, using text formatting options available
4. On the front-end, confirm the changes are reflected

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?

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