DUPP-32 Set `dp-rewrite-republish` as `'public' => false`

[enricobattocchi]

Context

• We want to avoid that posts with dp-rewrite-republish status are being displayed or even queried for in the front-end. Closes #211

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where Rewrite & Republish copies could be displayed and queried in the front end.

Relevant technical choices:

• Also fixing some CS warnings in related test.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• visit the frontend with Debug Bar and/or Query Monitor installed and configured
• check that the main query does not have OR $prefix.post_status = 'dp-rewrite-republish' in the WHERE
• check that the R&R feature still works:
  • create a R&R copy from a published post, change something and republish it (check the logs that everything goes right)
  • create a R&R copy from a published post, change something, schedule its republishing 2 minutes in the future, wait to see everything goes right.

Test instructions for QA when the code is in the RC

• [X] QA should use the same steps as above.

QA can test this PR by following these steps:

*

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

*

UI changes

• [ ] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• [ ] I have written documentation for this change.

Quality assurance

• [X] I have tested this code to the best of my abilities
• [ ] I have added unittests to verify the code works as intended

Fixes DUPP-32

[karlijnbok]

CR & Acceptance: 👍