Add a chat history screen to the im client use case

drafts/use_cases.rst

@@ -45,6 +45,7 @@ Home Screen
   - Message a user (with user ID)
   - Leave a recent room
   - Open a room
+  - Open a chat history link.
 
 Chat Screen
  What's visible:
@@ -103,6 +104,18 @@ Chat Screen
   - location
   - vcards (potentially)
 
+Chat History Screen
+ What's visible:
+  - The linked message and enough scrollback to fill a "screen full" of content.
+  - Each message: timestamp, user ID, display name at the time the message was
+    sent, avatar URL at the time the message was sent, whether it was a bing message
+    or not.
+  - The historical user list. *TODO: Is this taken at the linked message, or at
+    wherever the user has scrolled to?*
+ What you can do:
+  - Get older messages by scrolling up (scrollback)
+  - Get newer messages by scrolling down
+
 User screen
  What's visible:
   - Display name