luci-mod-admin-full: restore opkg feed config capability

opkg config was recently changed by https://dev.openwrt.org/changeset/46491/

Existing /etc/opkg.conf was split to three:
   /etc/opkg.conf -> base opkg configuration
   /etc/opkg/distfeeds.conf -> default Openwrt package feeds
   /etc/opkg/customfeeds.conf -> custom package feeds

Since then, the actual feed definitions have not been visible/configurable,
as only /etc/opkg.conf has been visible in Luci.

This patch restores the capability to see and edit package feed definitions.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>