luci-mod-status: improve channel_analysis page

luci-mod-status: improve channel_analysis page

Many user complained problem with using pool with wifi scan. This comes
from the limitation that some wifi driver have problems with scanning
nearby wifi and keeping traffic.

Fix this by doing the wifi scan only one time on page load and provide a
button to refresh the channels manually. The original implementation is
preserved as the user can simply reenable the poll referesh from the ui.

While at it also sort the table by channel instead of by signal quality
to better track the most used channels in the table.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
[rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>