This commit restores the proper setting of `$this->StoragePath`
using `GROCY_STORAGE_DIR` in `FilesService`.
The previous iteration of this patch mistakenly patches out the
setting of `$groupFolderPath` in the `GetFilePath` method, which
the method subsequently tries to return. This causes `GetFilePath`
to return, for example, `/file.png` (an incorrect root path) for
files instead of `/var/lib/grocy/userpictures/file.png`.
