Previously when system bots used to `#-mention` a private
channel's topics in cases like moving messages of a private
channel, then the #-mentions were not rendered by the
markdown. This was because the system bots did not have
authorization to mention these channels.
This is fixed by passing down an `acting_user` parameter
in code paths involving sending these move message
notifications so that permission of acting_user to mention
the topic is verified for rendering the markdown, rather
than that of the system bot.
4789de2e96