REST API: Add reader media to post endpoints

Adding media to /sites/$site/posts/$post endpoint enabling Reader
implmentations to get a single post from the API with the same
fields as /read/ endpoints.

Currently does not do anything in Jetpack.
REST API: Add /me/sites endpoint

Returns a list of the token user's sites. Site responses now have a
boolean visible property to indicate wether the user has hidden the
site from their list or not.
REST API: Adjust the way the $blog_id is set and used

In the /sites/ endpoint, adjust use of $blog_id so that it's properly
defined when fetching the subscribers count.
REST API: Several updates related to time and ordering

This includes:

r87548-wpcom:
    Update parse_date() to respect offsets that are specified within the
    given date string and cleanup parsing functions to use DateTime objects
    for consistency and explicitly defined timezones.

r87564-wpcom:
    Update comments for parse_date() method.

r87604-wpcom:
    Add post GUID as one of the response fields for the get/list
    posts endpoints.

r88186-wpcom:
    Add new order by field (ID) to list posts endpoint.
REST API: Add is_following to Site response

Also fixes a bug. get_current_user_id() can return 0 so use the
token_details to retrieve the users's id instead.

Merges r88955-wpcom.
REST API: Add site_id to author properties

For comments, likes, posts and other places an author object is embedded
if the user has a primary blog, its id is included as the site_id
property.

Merges r88959-wpcom.
REST API: Add 'site_ID' to the posts endpoints

Per request from @aerych.

Merges r88963-wpcom.
REST API: Rename site_id to site_ID

This merges r88970-wpcom.
REST API: Adjust several descriptions

This merges r89490-wpcom, r89492-wpcom, r89494-wpcom, and r89499-wpcom.
REST API: Type should be passed as an array

This merges r90281-wpcom.
REST API: Allow _thumbnail_id to be modified in postmeta

_thumbnail_id is a protected meta field and not editable by default.
This change whitelists _thumbnail_id in
WPCOM_JSON_API_Post_Endpoint::is_metadata_public() via the filter
rest_api_allowed_public_metadata making setting featured images via
the REST API possible.

This merges r90285-wpcom and closes #173.
REST API: Tracking code update

This merges r90402-wpcom, but this change doesn't affect Jetpack.
It is being merged solely to keep the files in sync.
REST API: Pinking shears

This merges r90664-wpcom.
Merge branch 'json-api-updates'

Merge REST API changes up to r90664-wpcom.