When a user is added to a channel, we send
the user that was added a Notification Bot
DMs to let them know about it.
In this commit, we add an option for whether or not
this message is sent.
If more than 100 users are added at once, we
do not send notification bot DMs since it would
be a performance-costly operation.
We also send this threshold value of 100 in the
initial state data to the clients.
Fixes part of #31189
5f80f0a970