Infinite Scroll: early set the DOING_AJAX constant.
Do it when the plugin is sourced -- don't wait until the `template_redirect` action.
Lets plugins / themes rely on the constant in prior actions (like `init`).
See issue: #1093
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
Featured Content: Correct check for whether or not the front page is the blog page.

Checking if `page_on_front` is empty can cause problems, since it's no longer empty as soon as a page is set once - even when the user changes back to using the blog as the front page, `page_on_front` retains the page ID. `show_on_front` is a more accurate way of determining if the blog is the front page or not.
Merge pull request #1098 from Automattic/fc-pre-get-posts

Featured Content: Correct check for whether or not the front page is the...
Featured Content: Validate show-all setting so it is properly saved.
Infinite Scroll: Make sure it's an XHR before setting DOING_AJAX early.
- Check the X_REQUESTED_WITH request header.
- Fall back to setting the constant in the response function if it's not already.
- Improve commenting
Merge pull request #1094 from Automattic/infinity-early-doing-ajax

Infinite Scroll: early set the DOING_AJAX constant.
Sharing: Use `photo` Twitter Card when `image` format and a featured image is set.
See: #1102
Sharing: Style fixes for #1103
In the code that sets a `photo` Twitter card type when a featured image is set on an `image` format post:
* strict equality checking
* 'yoda' conditional
* `} else {` on a single line
props @georgestephanis
Merge pull request #1103 from Automattic/twitter-photo-card-featured

Sharing: Use `photo` Twitter Card when post is format `image` & using a featured image
Reorganized CSS

Moved `scss/`, `img/`, and `css/` to root
Combined `img/` with `images/' and moved `images/` to root