luci-app-attendedsysupgrade: upgrade to version 2

Add required ubus acls from package rpcd-mod-attendedsysupgrade.

luci-app-attendedsysupgrade: add postinst script

removes caches and restarts uhttpd as mentioned here:
https://github.com/aparcar/gsoc17-attended-sysupgrade/issues/58#issuecomment-339370803

luci-app-attendedsysupgrade: redesign

simplified the interface and made it more "luci" like

simplify ubus interaction, use uci_call und ubus_call functions

add optional settings:

auto_search:
search on opening the view without clicking

advanced_mode:
if set, allow package edits, show build log

rename updateserver to upgradeserver
rename url to server_url

luci-app-attendedsysupgrade: rename uci options

remove leading "upgrade" from uci options
also rename server_url to url

cleaning

use new api/ urls

wait for successfull reboot

Signed-off-by: Paul Spooren <paul@spooren.de>