[Try] Move Subscriber management: move to wp-admin from Calypso/Jetpack Cloud

[jorgefilipecosta]

This is PR not intended to be merged moving subscriber management from calypso/jetpack cloud to wp-admin.
The idea is to iterate on something end-to-end here. But merge things step by step.
This is the intended merge plan:

• Boilerplate PR adding a new subscribers section, and the infrastructure to build a react page with dataviews under that page. The page will be under a feature flag.
• Initial backend PR expanding the REST endpoints to accomplish our needs.
• Start moving the UI implementation, without actions (multiple PR's in parallel).
• Implement the actions.
• Deal with complex used cases where calypso/wordpress.com is still required namely migrations of subscribers between different wordpress.com/jetpack sites, substack import, and CSV export.

Testing instructions:

Open jetpack go to "New subscribers" page, currently it should be possible to see a subscribers page similar to this:

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