A few small changes to the Discord Bot

[AlexanderHott]

These are small requests from @andreaskoepf to improve the discord bot.

• The timeout of the private bot-session is set too low, please make it configurable in seconds, default to 600.
• Please add a colon at the end of "Enter response here:"
• Please move task instructions like "Reply with the numbers of best to worst replies separated by commas (example: '4,1,3,2')" to the bottom.. e.g. below the ranking-options, so that the comma delimited example is visible just above the discord-input text-box at the bottom of the screen.

[andreaskoepf]

I would suggest to move the timeout parameters also into the settings class. But the PR can be merged so far.