Hide picked copies of non-public posts

If a node picked a non-public post from another node, it should hide it from everyone (except the node admin), because it does not know, who is allowed to view the post. Only the original node has this information.

Since the original post is non-public, it will not be visible in Web UI on the original node. Thus, any node that picked it should not show it in Web UI.

In the client, the node should display a placeholder ("A non-public post here") instead of the post and then query the original node whether the particular client is allowed to view. After the response, the placeholder should be or removed completely, or replaced by the post content. The result of the query may be cached then.

In REST API the query of such a post may return a special HTTP code indicating that the post exists but permission is pending. In a feed slice query there may be a special form of PostingInfo that designated such a post in the feed. When the permission is received, the client is informed by an event, then the client should query the post again to receive the actual content.

If the original node do not want anybody else to know, which users are allowed to view the post, it may simply return false to every permission query. This will make picked posts effectively private, but on the original node the original post will be visible to allowed users.

No files, tests, or concrete entry points are named. Start by reviewing the REST API, feed-slice PostingInfo, client permission-query event, and Web UI behavior described here; done means picked non-public posts are hidden, represented by the stated placeholder, and either removed or replaced after the original node responds.