Shared Avatar/Byline Formatters

[jeffersonrabb]

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:

Extract Avatar and Byline formatter functions to a shared file, which is access by Homepage Posts and Carousel.

Closes #370.

Note: Raised two ESLint warnings, which I'd like to resolve in a separate PR.

src/shared/js/formatters/index.js: line 15, col 4, Error - The href attribute requires a valid value to be accessible. Provide a valid, navigable address as the href value. If you cannot provide a valid href, but still need the element to resemble a link, use a button and change it with appropriate styles. Learn more: https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/anchor-is-valid.md (jsx-a11y/anchor-is-valid)
src/shared/js/formatters/index.js: line 33, col 6, Error - The href attribute requires a valid value to be accessible. Provide a valid, navigable address as the href value. If you cannot provide a valid href, but still need the element to resemble a link, use a button and change it with appropriate styles. Learn more: https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/anchor-is-valid.md (jsx-a11y/anchor-is-valid)

How to test the changes in this Pull Request:

1. Add Homepage Posts and Carousel to a post.
2. Verify identical bylines and avatars are shown in both.
3. Experiment with a variety of author configurations, including normal WordPress author, Co-Authors Plus mutliples, Avatars uploaded using WP User Avatar, etc.

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?

[adekbadek]

One discrepancy I've found - while in the editor multiple authors appear as group, with square avatars:

On the frontend there's just the first one, with a rounded avatar:

Should fixing this be a part of this PR?

[adekbadek]

Looks like the issue has been resolved by #455