Improve the way we display undelivered files attachments due to size

[mkdbns]

Right now, we display an ephemeral message in this case. However, the user may miss this leading to confusion. Also, we need to save the original post in case it gets updated on Teams later.

Propose to add the ephemeral text as a comment in the real post so that it is permanent. Have to make sure we don't remove it on edits though.