Configuration options for Navigation and FileManager/Browser

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).