Fix assorted bugs

[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:

Closes #278
Closes #246
Closes #279

How to test the changes in this Pull Request:

1. Use Posts Inserter in a Newsletter, observe maximum 20 posts can be insterted
2. Set an excerpt for a post, observe that Posts Inserter will now display the excerpt
3. Use a Heading block in a Newsletter, turn part of the text into a link
4. Observe the link color is same as text color
5. Set a custom text color to the Heading, observe the color is also set to the link

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?

[dkoo]

Looks 👍

[matticbot]

🎉 This PR is included in version 1.8.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀