gluon-luci-private-wifi: UI for enabling a private WLAN

This pull request contains the package `gluon-luci-private-wifi` for setting up a private WLAN as requested in freifunk-gluon/gluon#122

I tested it by running my device in config mode and copying the files with scp into to target folder:
`scp -r gluon/gluon-luci-private-wifi/files/* root@192.168.1.1:/`
I did not create a build yet.