Multiplayer improvements

by ilor for Battle for Wesnoth

The wesnoth multiplayer server is in need of some improvements to allow larger communities to participate. Even more so, the "lobby" user interface is largely inefficient and difficult to extend. This project aims to rewrite the UI part in the client for a smoother experience, and update the server-side code to match. Main expected features are a new game list with filters, a configurable room-like system, and moderation aids.