Jetpack Settings: Add actions, reducer, selectors and query components for Jumpstart

[tyxla]

Introduction

This PR introduces actions, reducer and selectors for managing Jetpack Jumpstart. Also, a query component for managing network requests for it is being introduced. This PR is part of the Jetpack Module Settings group effort (#9171).

New WP.COM API methods

For interacting with the .com API it introduces 2 methods that are not yet implemented on the API side:

• getJetpackJumpstart()
• updateJetpackJumpstart()

In order to fully test these methods you can:

• Apply D3588-code on your .com sandbox
• Apply Add Jumpstart status REST API endpoint jetpack#5813 on your Jetpack site (this PR is needed to enable the endpoint for requesting the current Jumpstart status, and to allow remote requests to the Jetpack REST API).
• Connect your Jetpack site to your .com sandbox and verify they talk to each other.

To test

• Checkout this branch
• Play with the QueryJetpackJumpstart query component and verify it dispatches the proper network requests - <QueryJetpackJumpstart siteId={ 1234 } />.
• Verify all tests are working: npm run test-client client/state/jetpack-settings/jumpstart/

/cc @oskosk @roccotripaldi @johnHackworth

[matticbot]

Test live: https://calypso.live/?branch=add/jetpack-settings-jumpstart-redux

[roccotripaldi]

LGTM! POST requests work as expected now.