`class.json-api-endpoints.php`: Resolve divergence from Jetpack.

This merges the following commits:
    - https://github.com/Automattic/jetpack/commit/5f7d9c5
    - https://github.com/Automattic/jetpack/commit/294bcc5
    - https://github.com/Automattic/jetpack/commit/1883b20
    - https://github.com/Automattic/jetpack/commit/2ce4133
    - https://github.com/Automattic/jetpack/commit/740917a
    - https://github.com/Automattic/jetpack/commit/0462475
    - https://github.com/Automattic/jetpack/commit/ea9d514

Merges r99709-wpcom.
REST API: allow titles in post references

See [98642]

Merges r99809-wpcom.
REST API: Add Jetpack Only endpoints file.

+ Add three new endpoints for managing plugins on Jetpack sites: list, activate and deactivate.
+ Trying to call the endpoint over wpcom sites returns an unauthorized error.
+ Jetpack only endpoints should extend the Jetpack_JSON_API_Jetpack_Only_Endpoint class..

Merges r99914-wpcom.
Merge commit '4384eba'

Conflicts:
	class.json-api-endpoints.php
Gplus: Typo fix.

See http://[private link]

Merges r99761-wpcom.
Shortcodes: Flickr: Add caching to the oembed handler

Our flickr handler doesn't use core's WP_Embed (which caches), so we need our own caching

http://[private link]

Merges r99856-wpcom.