REST API: Enable field filtering of Jetpack sites by casting object response to array

Includes (previously failing) test case
Issue: https://help.automattic.com/staff/index.php?_m=tickets&_a=viewticket&ticketid=1932282&departmentid=39&ticketstatusid=5

Merges r104553-wpcom.
REST API: Resolve empty response being stripped of all keys

Was previously treated as object response since `empty` would pass as true on empty response, and `$has_filtered` would only be set to true if response key was non-empty.
Ticket: https://help.automattic.com/staff/index.php?_m=tickets&_a=viewticket&ticketid=1932282

Merges r104645-wpcom.
Merge commit 'b8d8168'
REST API

Helper functions for loading a theme's functions.php (so things such as post formats, or CPTS can be loaded).
See #3046.

Merges r104371-wpcom.
REST API: Add support for "auto-draft" post status

This adds support for auto-drafts, which are temporary
placeholders created under the hood when the new post
page is loaded up, reserving a post ID so that e.g.
embed/shortcode previewing works.

Merges r104664-wpcom.
REST API

New endpoint for loading a theme's post formats. The current method (now deprecated - and will be removed in a vesioning) doesn't work properly.
This new endpoint uses [104371] to load a theme context so we always get the right set of formats.
This change is going to be used to fix https://github.com/Automattic/beep-beep-boop/issues/48 in the interim editor.

Merges r104372-wpcom.
REST API: Bypass looped call to get_taxonomy to avoid incurring additional queries

Both get_categories and get_terms return the same properties that the additional get_term_by query would return, so there's no need to make these calls. This should have a significant impact on performance by reducing the number of potential database queries from N (where N = # categories or tags) to a single query.

Merges r104721-wpcom.
REST API: Bypass looped call to get term details to avoid incurring additional queries

The detail provided by get_term_by is the same as what would be returned by get_taxonomy and wp_get_post_categories, so there's no need to make these calls. This should have a sign$

Related to r104721

Merges r104722-wpcom.
Add the Medium 'oemebed' and shortcode to Jetpack