Fix/1222 log post author change

[tharsheblows]

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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.

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