Some smaller todos

[AndreasGassmann]

• [x] Self-join counts as a join
• [x] "Time on server" in the leave messages is always a few seconds ago.
• [x] Option to disable/not count joins from users whose accounts were created less than _ time ago.
• [x] DMs don't work
• [x] Show prefix in welcome message
• [x] Add a message to be posted in #general after the bot was invited. It should be very short, only say `Your current prefix is !, see commands with !help, to set up join and leave messages do !setup)
• [x] We might want to make the !setup command easier and move the missing permissions to a !diagnose command (we can still internally call the !diagnose command every time someone does !setup)
• [x] {{prefix}} needs to be replaced in !memberconfig text

[Valandur]

Replaced by #102, #103, #104, #105 and #106