Update wordbot leaderboard and scoreboard to use "denotified" nicks

Nicknames that are sent by wordbot for the leaderboard command and the
end-of-game scoreboard are split up by zero-width space characters, to
avoid pinging the users at random times.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/util.ex

@@ -12,6 +12,6 @@ defmodule Omnibot.Util do
   create a notification for that user.
   """
   def denotify_nick(nick) do
-    Enum.join(nick, "\u200b")
+    String.graphemes(nick) |> Enum.join("\u200b")
   end
 end