Change "Unknown" lobby behaviour to be more compatible

Currently, lobbies are marked as Unknown when:

There are no mods installed on both the client and server
There are no incompatible mods on the client

This is the theoretically most correct way to do it - if you have an Everyone mod installed, it technically shouldn't be compatible with an unknown lobby.

However, it is very likely some people will be running exactly the same mod setups (some with LobbyCompatibility, some without) in the following situations:

The early transition period, when not everybody has updated
Any mods that use a soft dependency with an Everyone/ClientOptional value

At least for now, we should allow people to join Unknown lobbies to avoid any compatibility issues. Later on, this can likely be changed once we have a large amount of mods using LC.

The following need to be done: 1) Update LobbyDiff to ALWAYS label lobbies without LC as unknown, regardless of the client's mods. This is how it was done in one of my previous PRs, but any way to check "the server returned null for LC mods" would suffice.

        if (PluginDiffs.Count == 0)
        {
            _cachedResult = LobbyDiffResult.Unknown;
            return LobbyDiffResult.Unknown;
        }

        if (PluginDiffs.All(pluginDiff => pluginDiff.RequiredVersion == null))
        {
            _cachedResult = LobbyDiffResult.Unknown;
            return LobbyDiffResult.Unknown;
        }

2) The join block patches need to be edited to allow you to join Unknown lobbies.

Not strictly required, but would be nice UI to have:

Ideally, trying to join an unknown lobby (from the public lobby menu) would show some sort of warning that it could be incompatible.
When viewing the modlist for an unknown lobby, instead of just showing the client's mods, some sort of text like "Could not find mod data for lobby" should be shown.

MaxWasUnavailable / LobbyCompatibility

Change "Unknown" lobby behaviour to be more compatible #32