Connection flows: improve the plugin experience for non-admins

[jeherve]

Non-admins currently get a sub-par experience when navigating Jetpack wp-admin screens or trying to use Jetpack features. Let's improve the experience for them whenever possible.

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1. Jetpack menu: only show to contributors and above. #39081
   Admin Page [Package] Admin Ui [Plugin] Jetpack [Pri] Normal [Status] Proposal [Tests] Includes Tests +6
   
2. [Deprecate Dashboard] React error and blank page when trying to disconnect (as non-admin) #34535
   Connect Flow Triaged [Package] My Jetpack [Plugin] Jetpack [Pri] Normal [Type] Bug +6
   
3. Publicize: editors cannot connect an account #38335
   Triaged [Feature] Publicize [Platform] Atomic [Plugin] Jetpack [Pri] Normal [Type] Bug +6