Publicize: Add a Jetpack Publicize connections endpoint.

[pablinos]

This adds a new endpoint, that proxies a request to WPCOM in order to retrive the Publicize connections. It also makes a change to the initial state, so that on WPCOM we call the API directly. It might be better to just have an isWPCOM boolean so that it's more useful.

Fixes #

Changes proposed in this Pull Request:

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

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