fix: memory leaks from legacy term utilities

[dkoo]

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Changes proposed in this Pull Request:

Fixes and improvements coming from feedback from the v2 beta test. Related issues reported in #76.

How to test the changes in this Pull Request:

Legacy term conversion memory leak

In v2+ of Listings, listing CPTs use regular post categories and tags instead of separate custom taxonomies. We have a utility in place to check for and convert those legacy custom taxonomies, but it's an expensive operation. This PR avoids running the legacy term conversion script on every admin_init hook. Instead, moves the conversion to a WP CLI command so it has to be executed manually.

Addresses #76 (comment).

1. On master, create some legacy Listing Categories and Listing Tags and assign to some listings.
2. Check out this branch.
3. Run wp newspack-listings taxonomies convert --dry-run while SSHed into your test site. Confirm that the script iterates through your legacy categories and tags and prints info on each, but doesn't persist any changes.
4. Run wp newspack-listings taxonomies convert. This time confirm that the legacy terms are converted to regular post categories and tags, and that the converted post categories and tags are applied to the same listings that previously had the legacy terms.

Missing/orphan shadow term memory leak

Shadow terms are what we use to assign listings to other listings and posts/pages as related content. It's the most performant way to associate posts with other posts in WP. Unfortunately, terms sometimes go missing, or their posts get unpublished or deleted, resulting in orphaned terms. We have some utilities in place to check for and fix these missing/orphaned terms, but they are expensive operations. This PR avoids a memory leak caused by the functions that create missing or delete orphaned shadow terms by moving those to a WP CLI command.

1. On this branch, publish some listings of any type, e.g. a Place. The plugin should create a shadow term for each place when it's published.
2. Using WP CLI, look up the shadow terms using wp term list newspack_lstngs_plc. You should see a table of the shadow terms for your published places, e.g.:

+---------+--------------------+--------------------+-------------------------+-------------+--------+-------+
| term_id | term_taxonomy_id   | name               | slug                    | description | parent | count |
+---------+--------------------+--------------------+-------------------------+-------------+--------+-------+
| 488     | 488                | A New Place        | a-new-place             |             | 0      | 1     |
+---------+--------------------+--------------------+-------------------------+-------------+--------+-------+

3. Delete one of the terms using wp term delete newspack_lstngs_plc <term_id>. This will result in a Place with a missing shadow term.
4. Create an orphaned term by using wp term create newspack_lstngs_plc Orphan --description="An orphaned term.". This will result in a shadow term without a corresponding Place listing.
5. Run wp newspack-listings taxonomies sync --dry-run. Confirm that the script identifies your missing and orphaned terms, but doesn't actually make any DB changes.
6. Run wp newspack-listings taxonomies sync. This time confirm that the missing shadow term is created and the orphaned shadow term is deleted (using wp term list newspack_lstngs_plc again).

Other information:

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[adekbadek]

Works as described 🙌

[matticbot]

🎉 This PR is included in version 2.0.0-alpha.2 🎉

The release is available on GitHub release

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