[MM-23727] Make channel validation consistent on the server (#14230)

* MM-23727 Ensure user ids not allowed in channel name:

MM-23727 Move channel name validation to model level

* MM-23727 Update wording

model/channel.go

@@ -234,6 +234,11 @@ func (o *Channel) IsValid() *AppError {
 		return NewAppError("Channel.IsValid", "model.channel.is_valid.creator_id.app_error", nil, "", http.StatusBadRequest)
 	}
 
+	userIds := strings.Split(o.Name, "__")
+	if o.Type != CHANNEL_DIRECT && len(userIds) == 2 && IsValidId(userIds[0]) && IsValidId(userIds[1]) {
+		return NewAppError("Channel.IsValid", "model.channel.is_valid.name.app_error", nil, "", http.StatusBadRequest)
+	}
+
 	return nil
 }