Reduce the amount of unnecessary messages shown in the game chat

Reduce the amount of unnecessary messages shown in the game chat

Some engine messages are being printed to chat in scenarios where they may obscure regular game chat.

To help with this, I've adjusted the visibility of them:
- "unconnected changed name to BOT" and "BOT left the game" showing in chat have been disabled by only allowing such messages to show for real clients
  Bots will still show that they've been dropped in the console and Xonotic still shows a (dis)connect message
- Verbose "Server lag report" messages (most commonly seen during map votes) have been moved to the console