Better placeholders for deleted or unavailable statuses

Follow up to #3 (comment 75023), an improvement upon !40 (merged). The current implementation can be enhanced by the actual tombstone activities.

The main issue with the current implementation is that we don't know if replies to non-existing posts are replies to the same post or different ones because inReplyToId is empty when the parent is not available. The replies to posts from blocked instances have the same issue.

Tombstone metadata might provide the users with context as to why the post is unavailable. E.g.:

• "This Status was deleted by its author"
• "This Status was deleted because it violates the Terms of Use of the instance"
• "This Status is unavailable because it comes from a blocked instance"