Unsubscribe from content listing page. [#3565697]

Problem/Motivation

When a user subscribes to content from a hub, and the content is configured to update automatically whenever the hub content changes. The user can delete that content from the content listing page without unsubscribing from the hub. This leaves the subscription out of sync. As a result, when the hub content is updated, the deleted content is re-imported into the client site, effectively undoing the deletion.

Steps to reproduce

Download or subscribe to some content. Delete it from the content listing page. Update the content on the hub site. See that the content has returned to the client site.

Proposed resolution

I think we should add an unsubscribe operation link on content that is "subscribed to" or pulled from a hub. We should also hide the delete operation until the content has been unsubscribed from.

Issue fork entity_share_websub-3565697

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3565697-unsubscribe-from-content changes, plain diff MR !28
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