Move the default profiles to the user’s home

Rather than using `/nix/var/nix/{profiles,gcroots}/per-user/`, put the user
profiles and gcroots under `$XDG_DATA_DIR/nix/{profiles,gcroots}`.

This means that the daemon no longer needs to manage these paths itself
(they are fully handled client-side). In particular, it doesn’t have to
`chown` them anymore (removing one need for root).

This does change the layout of the gc-roots created by nix-env, and is
likely to break some stuff, so I’m not sure how to properly handle that.

src/libstore/local-store.hh

@@ -281,8 +281,6 @@ private:
     void signPathInfo(ValidPathInfo & info);
     void signRealisation(Realisation &);
 
-    void createUser(const std::string & userName, uid_t userId) override;
-
     // XXX: Make a generic `Store` method
     FixedOutputHash hashCAPath(
         const FileIngestionMethod & method,