Traffic graph/widget - Remember user choice of bits/bytes between uses

Currently lost and must be re-clicked every time traffic graph is viewed.

Also likely to add so it remembers scale setting too.

Notes:

    1) "bandwidth_by_ip" isn't used elsewhere, so moving unit labels ("bits/bytes") here appears safe.
    2) Is "null_call()" in "graph.php" correct Ajax arg when no response is needed.  It works though.
    3) Check needed in case 2 files with the same names in "pfsense_packages/config/rate" need matching changes.  ("bandwidth_by_ip" and "status_graph.php" exist in two places)