Save Replays in per Mod and Version folders

as discussed in #2152 this will save replays in $HOME/.openra/Replays/$mod/$version instead of putting it all into one folder. That has several advantages:

* No more https://github.com/OpenRA/OpenRA/wiki/FAQ#old-replays-crash-the-game reports.
* Faster loading in the replay browser because fewer files need to get parsed.
* Easier to clean up and maintain that folder as it can grow huge.