Rework server browser to prioritize ETL default mod servers

[rmarquis]

As part of our effort to make it friendlier to search for a valid servers, rework the Legacy mod server browser to:

• display ETL servers as priority, by implement a "light" server browser listing Legacy mod servers only.
• display running version
• add filter to display only server running the latest release
• ensure users can still select any server running a third-party mod.

Other improvement might include:

• adding a "Quick Join" feature that would select a populated server based on availability or popularity.
• add blacklisted/recently visited servers. See Add blacklisted and visited servers in server browser #888.

[Calinou]

This issue is a few years old, but I think that as of the latest release, it might make sense to default the mod filter to Legacy for a few reasons:

• A lot of popular servers run the Legacy mod now. Server population on Legacy mod was an issue a few years ago, but it's hardly ever a problem since ET's release on Steam prompted many servers to move to the Legacy mod.
• The Legacy mod has most features you'd expect from a modern mod, with no intrusive gameplay changes by default.
• The Legacy mod is one of the few mods that works on x86_64 builds (which many Linux users might be using, as it's a bit easier to get running). If we want to provide Windows x86_64 builds (which can improve performance in CPU-bound situations), this will start to matter on Windows too.

Of course, the mod filter remains available to change at any time in the server list, so this is about providing a better experience for new players. It may also be worth enabling the filter to exclude bots from player count sorting by default.