feat(welcome-message): Welcome message

[Jan]

Is your feature request related to a problem? Please describe.
Some servers want to welcome their new members with a wholesome message. Such messages often list important channels and a quick introduction to the server.

Describe the solution you'd like
Server Admins can set a message that is automatically sent to either the new member or to a new channel.
The message should allow variables:

• username
• server name
  The message should be an embed having a default layout, that can be customized too.

Describe alternatives you've considered
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Additional context
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[Konstantin W.]

Thanks!
This is also something that could later be setup/customised using the Dashboard (see #65).