luci-mod-network: wireless.js cell_density support

Add support for the cell_density option added to OpenWRT via commit:
81ff23fc91dbbedc374e58afdb4b0b13146c0f15

This configures data rates based on the coverage cell density.
Normal configures basic rates to 6, 12, 24 Mbps if legacy 802.11b rates
are not used else to 5.5, 11 Mbps.
High configures basic rates to 12, 24 Mbps if legacy 802.11b rates are
not used else to the 11 Mbps rate.
Very High configures 24 Mbps as the basic rate. Supported rates lower
than the minimum basic rate are not offered.

Signed-off-by: Nick Lowe <nick.lowe@gmail.com>