Add Config options for File Manager/Browser and Navigation app Paths.

Resubmitted as the `conf/config.php` file was missing - sorry!

Allows the file used to store the Navigation app menu to be set by a configuration option:-
In conf/config.php or the file set by ELEFANT_ENV :-

[Navigation]
json_file = /conf/mymenu.json

will make Elefant use the menu data from mymenu.json instead of the default navigation.json.

Also, allows the directory where Elefant stores files/images can be configured:-
In conf/config.php or the file set by ELEFANT_ENV :-

[FileManager]
root = /myroot/

will make Elefant use files/images from /myroot/ instead of the default /files.

Needs testing of Aviary.com online (as opposed to private dev server).