Send challenges silently

[albertoxamin]

There are cases in which the bot is added to very large groups that need to have notifications on. It would be beneficial if in such groups the bot could send messages with the silent feature in telegram. So when a new user joins it wont bother thousands of the group's members. This option needs to be passed according to the telegram bot apis disable_notification

https://core.telegram.org/bots/api#sendmessage