read_receipts: Disable showing read receipts for Notification Bot msgs.

The implementation is simple, we just check if the
the message sender is a notification bot to decide if we
should show the read receipts list.

We also update the modal content styling to match the padding at the
top of the modal.

Fixes #22905

tools/lib/capitalization.py

@@ -109,6 +109,8 @@ IGNORED_PHRASES = [
     # Use in compose box.
     r"to send",
     r"to add a new line",
+    # Used in showing Notification Bot read receipts message
+    "Notification Bot",
 ]
 
 # Sort regexes in descending order of their lengths. As a result, the