This is the first step toward bundling all assets related to a save game
into a single item. That makes it easier to avoid clobbering "temporary"
assets from other games like the state.json, but also makes it easier
for players to file bug reports, since there's only a single asset to
upload.
This is only the first step because so far it only includes the various
save files: start of turn, end of last turn before results processing,
and "latest" (the game saved explicitly by the player).
We often get save games uploaded with bug reports that are already in a
broken state with nothing we can do about it. This saves that turn to
`last_turn.liberation` so users are less likely to have clobbered the
useful data before filing the report.
