Fix/1222 log post author change

[tharsheblows]

Fixes #1222

This changes the hook used for watching for post updates to post_updated and uses set_object_terms to watch for term changes. The idea is that everything which triggered or should have triggered a log previously still triggers a log but with more information.

The strings in the summary are built separately and then the data itself is stored in an associative array in the meta table. (nb: this should be a JSON object for cross language portability I think)

Currently tests are failing as I'm not convinced this is completely the right approach.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

[delawski]

💡 thought: I believe this will not log new posts creation because post_updated is only called for existing posts.

It may be a good thing because I think we don't need to log e.g. a post_title change for a new post. However, without transition_post_status callback we no longer log new posts creation. I guess we could use e.g. save_post action or bring back transition_post_status but limit its scope to only log transitions to the publish status.

[tharsheblows]

@delawski $update is a check on whether or not the data has a post_id which gets set immediately in the editor but ... if someone is using wp_insert_post() directly to add a post then it won't so I need to add that in, thank you!

[tharsheblows]

@delawski I've changed this to use wp_after_insert_post -- it's noisier but because of the orders of the filters in the REST API, it's the one that works there as well as in the Quick Edit section.

I still need to test and add automated tests though!