Until now, migrating would always attempt to move files.
There's a chance that a user is reinstalling osu! but has their data at
a custom location. We want to allow the chance for them to continue
using the external data. This seems like the easiest way to make it
work.
Would be nice if we had a `Game.Restart()` method, but maybe this is
enough for now?
Note that further down the road we will probably prompt the user to
potentially select a custom install path (including one with existing
data) before osu! gets to writing anything.
