luci-base: network.js: introduce further API functions

luci-base: network.js: introduce further API functions

 - Add Network.getWifiNetworks() to simplify fetching *all* networks
   instead of building a list by iterating each radio first

 - Add Network.WifiNetwork.isClientDisconnectSupported() to check whether
   a wireless network supports client disconnections

 - Add Network.WifiNetwork.disconnectClient() to trigger client
   disconnections via hostapd

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit a2dc6c8216e695d758a3667a9dbc299188b423cd)