REST API: add new query paramater for filtering the fields returned

This query paramater allows you to specify which fields to return with endpoint responses by passing an additional comma-separated list of fields as part of the query.

For endpoints that contain one of the following objects: categories, comments, connections, domains, groups, likes, media, notes, posts, services, sites, suggestions, tags, themes, topics, users -- it will filter the contents of that object rather than the "parent" object. Otherwise, it filters the main returned object.

Comes with tests to make sure the filtering works correctly. Also removes an extra debug that was left in one of the tests.

see https://[private link] for project details

Merges r96761-wpcom.
REST API
Likes

Just check the setting directly. Jetpack_Likes will fatal on JP blogs if we don't load a file. Instead of loading a whole file just for this line, check the option.

Merges r96763-wpcom.
Merge pull request #503 from Automattic/json-tweaks-justin

Some REST API tweaks from Justin