REST API: Include post title in comment response

Merges r98642-wpcom.
REST API

Whitelist publicize related meta for developers to use.

Merges r98692-wpcom.
Media Extractor: Toss out extracted image urls that are over 4kb.

These are likely either data URI encoded images or they are invalid / from
malformed html so we want to ignore them.

Merges r98495-wpcom.
Media Extractor: Toss out extracted URLs that are over 4KB

These are likely either data URI encoded images or they are invalid / from
malformed html so we want to ignore them.

Merges r98501-wpcom.
Media Extractor: Missing required param of already extracted items.

Merges r98662-wpcom.