Sharing: Add Feature Gate Component

[v18]

The Sharing section needs a feature gate in case the site doesn't meet the necessary criteria. For example, we need to display a feature gate for Jetpack sites that do not have Publicize or Sharing enabled.

Here's a mockup for a similar feature gate for Jetpack Manage:

We should also show a good error screen for sites that do not have sharing enabled, but still end up on this page. Some examples of that:
(1) some WordPress.com sites
(2) visiting the sharing page as a contributor to a site

Here are more details for contributors specifically: Contributors can view shared publicize connections but cannot add new connections, have their own connections, or in any other way interact with connections. In addition, they cannot view/interact with non-publicize connections (e.g. eventbrite).

Currently, this is what contributors see in Calypso's sharing screen by entering the URL directly in their browser:
https://wordpress.com/sharing/example.com

[stale]

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