improve(leaderboard): close file handles

fix $11e - add random motivation texts at random channels (from a certain list of channels). Random texts can appear every 1 to 24 hours

fix $125 - Allow up to 18 possible answers for a poll => cap of reactions that can get added to a message

fix: $121 - fixed issue, that also sent messages, where the bot was the author, into the support channel

fix: $122 - added greeting message, that is sent as soon as somebody is assigned the "Studentin" role