Auto sync parent terms

[MARQAS]

Description of the Change

Auto-sync the parent terms when child terms are selected in the posts

Closes #3105

How to test the Change

• Go to WordPress default categories
• Add 2 terms, make one of them child to the other
• Go to WordPress default posts
• Add a new post and link the child term with it
• Edit the parent term slug
• Go to Elasticsearch and check the post document and note that parent slug is also updated

Changelog Entry

Fixed - Auto sync issue with parent term

Credits

Props @MARQAS

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@MARQAS a couple of questions:

1. Can we add some unit tests for this?
2. Do we also have the bug when a parent term is deleted? If so, this covers that scenario as well?
3. To increase readability, do you mind wrapping variables in double quote strings with {}, like in "SELECT object_id FROM {$wpdb->term_relationships} WHERE term_taxonomy_id...
4. Will we have a performance problem if too many posts are associated with child terms?

[felipeelia]

@MARQAS do you mind making the suggestions I've added? Just so we keep var names consistent. Thanks!

[MARQAS]

@felipeelia I have added the changes you asked for along with a notice. I observed that it wasn't showing notice when the parent term was not attached to the posts.
Steps to reproduce:

• Add parent and child terms
• Attach child term to the posts
• Make Content Items per Index Cycle less than the posts attached to the child term.
• While editing a child term, you can see the notice.
• But it will not show the notice when editing a parent term.
• It will only show the notice on the parent term if the posts attached to the parent term are greater than Content Items per Index Cycle

With this PR, it will show the notice on the parent term as well if posts associated with one of its child terms are greater than the Content Items per Index Cycle