luci-app-statistics: add support for apcups plugin

OpenWRT/LEDE support for APC UPSes is only partial: although the collectd
apcups plugin is included, related lua/luci code is missing. These changes
add the lua side and have been used for ~2 years, both on OpenWRT and LEDE.

Reworked from patches submitted by James Klaas to the luci development list
in 2015.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>