luci-mod-admin-full: use incremental background scanning for wireless join

The previous approach of synchroneously scanning while building the result
page was suboptimal since it frequently led to connection resets when
accessing LuCI via wireless.

It also exhibited problems when accessed via SSL on recent Firefox versions
where the page were only loaded partially.

Rework the wireless scanning to gather scan results in a background process
and put them into the ubus session data area where they can be readily
accessed without causing network interruptions.

Subsequently rebuild the wireless join page to use XHR polling to
incrementally fetch updated scan results.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>