"Headless" threads do not show in the client sidebar

[mkdir-washington-edu]

What is the issue? Right now the behaviors are - Reply to a deleted reply:

• The deleted reply is replaced by "Message not available."
• The reply to the deleted reply is still available in the client sidebar.

Reply to a deleted top-level annotation:

• The deleted top-level annotation is not visible
• The reply to the deleted top-level annotation is not visible in the client sidebar.

We should make these behaviors the same. Particularly for users who have replied to a now-deleted top-level annotation, their comments and thoughts are no longer available to themselves or other users. To solve these issues, we could make the behavior of replies to deleted top-level annotations match the current behavior we use for a reply to a deleted reply.

To Reproduce this behavior

1. Create a top-level annotation.
2. Create a reply to the top-level annotation
3. Create a reply to the reply
4. Delete the reply to the top-level annotation. Note that the reply to the reply is still available on the page.
5. Delete the top-level annotation. Note that the whole thread disappears.

Screenshots

Additional context https://hypothes-is.slack.com/archives/C03QZM0K4/p1589483503264100