CSS Editor: change "Content Width" label to "Media Width".

Even though the variable is named `$content_width`, the actual width of content is not changed by that setting from a user's perspective. It is more clear to say "media width" to users.

Fixes #6444.

Merges r109202-wpcom.
Merge commit 'fa7ef36'
REST API: Making it possible to add new objects to cast_and_filter_item, without adding them to files that are shared with Jetpack.

It would be nice to do this using apply_filter, but that method doesn't seem to work with the test suite.
See https://[private link]

Merges r109103-wpcom.
REST API: Merges r109137-wpcom.
REST API: Add `parent_id` argument option to post listing endpoint

Merges r109155-wpcom.
REST API: Init v1.1 of the post endpoints.

The new 1.1 files will load when /v1.1/sites/$site/posts/* is called. v1 will continue to serve the old code.

Merges r109258-wpcom.
REST API: adds restore endpoint for posts/pages in trash

Merges r109268-wpcom.
REST API: Add `exclude` argument to post listing endpoint

For both v1 and v1.1 endpoints.

Merges r109311-wpcom.
REST API: Add `exclude_tree` argument option to post listing endpoints

For both v1 and v1.1 endpoints.

Merges r109331-wpcom.
REST API: Updates the example response for post endpoints

Merges r109430-wpcom.
REST API: fixing typo in example response

Merges r109431-wpcom.
REST API: adding capabilities endpoint to post object

Merges r109310-wpcom.