Support swapping of existing Ghost files into the fresh install

[gingeleski]

Add optional logic for swapping existing Ghost files into the fresh install.

I foresee this working like there's a full Ghost content/ dir in the root of the project here that gets swapped into the fresh install.

If there is not a SQLite database inside then we might also do a check for a JSON export in the project root, which we could programmatically import via ghost-cli.