Persist skirmish settings between sessions

[pchote]

Closes #4122
Closes #4787

[pchote]

Rewritten to split the skirmish logic from LobbyCommands into its own ServerTrait.

[PunkPun]

IsSkirmish should be true when in lobby from the map editors PlayMap option

[pchote]

IsSkirmish should be true when in lobby from the map editors PlayMap option

No, as I mentioned above, this must be false to prevent the server logic from overriding the map being edited, and for the other state set in the editor from overriding the skirmish data.

[RoosterDragon]

Overall seems good.

If I modify the saved file to create an invalid option (e.g. set gamespeed: no-such-option), then I can no longer open the Skirmish menu without an error

I don't think we need to protect against users messing with the file as such, but in the future if we change the lobby options or anything else that gets saved then the file on disk might not be valid any more. I think we need to recover from this scenario. Whether we drop the bad file entirly or try and recover individual options in a piecemeal fashion I don't mind.

[pchote]

Rebased and fixed.

[PunkPun]

Overall LGTM

[PunkPun]

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