Handle column middle alignment

[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:

As discussed in #231, column middle alignment is a tricky issue because Gutenberg uses flexbox, and MJML uses vertical-align CSS property. Recreating flexbox somehow is a bigger undertaking, so as a compromise the possibility of an all-column-middle-aligned layout is introduced in this PR. Before, middle alignment of columns was not possible in any configuration.

Closes #231

How to test the changes in this Pull Request:

1. Create a newsletter, insert a columns block with some columns
2. Set the columns block to middle vertical alignment (the content should be of different heights for the effect to be visible) – send a test email and observe the content is indeed middle-aligned, as in the editor.
3. Change one of the columns to a non-middle alignment – observe a warning being shown.
4. Fix the error by either setting the columns to middle alignment one by one, or by setting middle alignment on the columns block

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?

[matticbot]

🎉 This PR is included in version 1.3.0 🎉

The release is available on GitHub release

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