feat: exclude current post in homepage posts block

[jeffersonrabb]

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• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
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Changes proposed in this Pull Request:

Exclude the current post ID in Homepage Posts block. This change allows placement of the block in content without the obvious mistake of displaying the current post.

Note: There are some unrelated issues that will occur when the Homepage Posts block is used in a Campaign. These will be addressed in a future PR.

Closes #359

How to test the changes in this Pull Request:

1. On master, create a new Post, add Homepage Posts block. Publish the post then refresh the editor page. Observe the current page is displayed in the block.
2. View the published post. Observe the current page is shown in the block in frontend as well.
3. Switch to this branch. Observe the current page is no longer visible in editor or frontend.
4. Verify "Show More Button" works properly and doesn't show the current page. Check this in AMP and non-AMP.
5. Verify complex homepage layouts continue to work normally.

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?

[claudiulodro]

Looks solid and works well 👍

[dkoo]

@jeffersonrabb some issues I ran into when testing:

1. With AMP enabled, if I enable the "Show 'More' button" attribute, the block in the front-end AMP post doesn't show any content, including the "More" button. This isn't a problem when AMP is disabled for the post. This is happening on the master branch, too, so maybe it's something with my setup and I won't let it block approving this PR.

2. The post isn't excluded from the block preview in the "styles" editor sidebar:

3. On the homepage, I couldn't get the latest post to appear for some reason. It appears correctly in the editor (note the "My Best Post" entry in the upper-right): This is happening on the master branch, too, so maybe it's something with my setup and I won't let it block approving this PR.

But not on the front-end (in fact Events seems to show different posts entirely—maybe something to do with sort order?):

Tested in both AMP and non-AMP, both logged-in and not logged-in.

For the crossed-out issues, if you don't see the same behavior, I'll consider it user error on my part.

[jeffersonrabb]

Discussed with @dkoo and we're going to investigate his issues in future PRs. Two of the are proving difficult to reproduce and the third (The post isn't excluded from the block preview in the "styles" editor sidebar:) is a very good point but will require some rethinking of the deduplication code (which intentionally ignores block previews).

[matticbot]

🎉 This PR is included in version 1.9.0 🎉

The release is available on GitHub release

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