Fallback for incomplete threads

When a user is invited to a restricted room that already has threads and messages, what should their experience be?

### Discussion:
- Users should be able to see messages that they have permission to see.
- Users may not be able to see Threads from the Thread Summary on the Timeline (because they do not have access to the Root Message)
- Users should have access to the Thread from the Thread List view
- Users should only see a Thread in the Thread List view if they have permissions to see at least one response (if all messages in a Thread are restricted to them due to permissions, it should not appear in the Thread List view)

### Needs:
**The user has access to the Thread messages (they have access to) from the Thread list view;**
- [x] 1. How does the Thread Summary in the Thread List view look
- [x] 2. What message do we share with the user when they don't have access to the root message

**When the user has clicked to view a thread that they only have rights to see part of;**
- [x] 3. How does the Thread look like (if the user cannot see the Root message, or the messages following...)
- [x] 4. What message do we share with the user when they are viewing the Thread but have access to view only a few of the most recent messages

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### Design

Figma: [Restricted History Threads](https://www.figma.com/file/T309ztx0sNyOOK6NKVLHsK/Threads?node-id=2761%3A308969).