Allow the filesystem backend root location to be configurable

Currently, the filesystem backend is always located at ~/.cristal.
This can be annoying for users.
Various use cases:

• For users: being able to have different local wikis for different domains
• For devs: being able to define a temporary location for functional tests (see CRISTAL-281) that we know to be empty at start, and that we can safely wipe after the tests

PS: The same is also true for the Nextcloud backend.