Implement /task conversation command

[Sebastian Oßner]

Implement a conversation with an entry point in /task. If the calling user is a game master, this should be interpreted as wanting to create a task and should include the following steps:

• If there is an ongoing task, terminate that and exit the conversation
• Otherwise, get the assignment of the task (e.g. "Enter the price of a xxx")
• Get the Regex of the answer (e.g. 12[.,]34€?)
  If an assassin calls the command, do this:
• Send the task assignment to the user again and ask for the answer
• check if the answer the user entered matches the Regex