more compact box for nick event

[hahahahacker2009]

Hydrogen has a really big box for nick event. The height of the box is 2x the height of the text. When there are many people join and leave, this would blown the message.

On element the height for nick event fits the height of the text, and I think it is 0.5x the height of message (IRC style).

The solution is to reduce the height of the nick event box to fit the text.