luci: initial 802.11ac support

luci: initial 802.11ac support

While working with 802.11ac (ath10k) I've noticed the web interface
configuration missing basic support for 11ac devices - unable to set VHT
(htmode) and 11ac (hwmode).

This patch adds initial support for luci admin-full page and 802.11ac
MAC80211 devices.

v2:
* replace obsolete 11nac mode with 11a + vhtmode (jow in ticket: #17323)

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
[jow: fix typo in get_i18n()]
Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>