REST API: abstract do_not_document check into a method

So we're not repeating code.

https://[private link]

Merges r106846-wpcom.
EST API
Media Endpoints (v1.1)
See https://[private link]

Introduce get_media_item_v1_1 (related to original get_media_item) that has an updated and cleaner/consistent structure (matches other endpoints where ID is capitlized, etc).
To be used in the v1.1 endpoints.

Merges r106873-wpcom.