Allow sending messages to channel from a client that is not a member of a channel

[mxafi]

RFC 2811 defines a channel mode n:
No Messages To Channel From Clients On The Outside

When the channel flag 'n' is set, only channel members MAY send
messages to the channel.
This flag only affects users.

We don't have to implement this mode per the subject requirements.
Our current implementation inherently implements this mode, without advertising it to the clients using our server and without any ability to toggle it by channel operators.

[mxafi]

This would require refactoring the current implementation, and adding a new channel mode n.