Add terms to return value in existing Post backup endpoint

[thingalon]

When fetching a post object backup, include linked terms via the term_relationships table.

Changes proposed in this Pull Request:

This PR adds a terms key to the result of the Post Backup API endpoint, which includes a summary of the linked term relationships associated with the requested Post Object.

Reference: pa0RFL-jR-p2

Testing instructions:

Execute the post backup endpoint via the wpcom API, and verify that the resultant output includes associated term relationships.

See pa0RFL-jR-p2 for more details.

Proposed changelog entry for your changes:

• Include updates to term relationships when backing up Post object changes.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: July 2, 2019.
Scheduled code freeze: June 25, 2019

Generated by 🚫 dangerJS against fd6f56d

[seear]

Looks good.

I tested from my VaultPress sandbox also using wpcom patch D29457 D29460 and saw the new terms field:

    "terms": [
        {
            "term_taxonomy_id": "2",
            "term_order": "0"
        }

[jeherve]

This seems reasonable. Merge when ready.