luci-base: support disconnecting (deauthenticating) wireless clients

Add a button to each row in the wireless assoclist table to allow
disconnecting clients using the ubus del_client method if the underlying
radio interface supports it.

Ref: https://github.com/openwrt/luci/pull/2271
Submitted-by: Wang805447391 <805447391@qq.com>
[move deauth function to luci-base next to the existing assoclist function,
 require post security, fix parameter check condition, hide button if not
 supported by the radio, disable button after call, squash commits, fix
 whitespace, reword subject, add commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>