My Jetpack: handle redirect when no connection

[retrofox]

Follow up of #22530

This PR adds changes about how the app handles the redirect when it doesn't have a site and a user connected.
Relevant points:

• It adds an optional redirect option to the useMyJetpackConnection() hook. When it's true, the client will perform a redirect to the Jetpack dashboard in order to offer the user to connect to their site.
• it checks at top of the app whether there aren't site and user connected. If so, it won't render anything and wait for the client redirect.
• Because of the above, I've removed the redirecting handling from the <ConnectionsSection /> component.

Fixes #

Changes proposed in this Pull Request:

• My Jetpack: handle redirect when no connection

Jetpack product discussion

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

No

Testing instructions:

• Check connecting and disconnecting the site.
• Confirm when it's disconnected, the app performs a redirect to the Jetpack dashboard
• In short, disconnect, connect, etc should work as expected

[github-actions]

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[leogermani]

Tests well in all situations

• fully connected
• connected at a site level
• redirect after disconnect