REST API: Allow clearing of tags or categories by sending empty field value

Previously, an empty field value would be ignored. This uses the behavior of assigning the default category if one is not set.

See: https://github.com/Automattic/beep-beep-boop/issues/90

Merges r104849-wpcom.
Open graph: allow the featured image to be used for the `og:image` tag

Merges r104874-wpcom.
Media Summary/Enhanced excerpt: set defaults for the number of max characters and max words so that they can be overidden on a case by case basis.

This will be used to fix the incorrect count sometimes generated in Facebook publicize messages.

Merges r104815-wpcom.