Zany logon/logoff announcements

Currently we have

• SYSTEM: welshbyte has just entered the board.

But that could have a bit of fun injected into it by generating random (welcoming!) entry announcements, e.g.

• SYSTEM: You rolled two sixes! welshbyte has just entered the board.
• SYSTEM: welshbyte has just pirouetted onto Milliways - how graceful!
• SYSTEM: The prophesy has come true, welshbyte has arrived.

(Inspired by Discord)