luci-app-opkg: show disk space consistent with overview

* Show Disk space graph consistent with the Status->Overview page
* Brought up in https://forum.openwrt.org/t/software-space-is-going-to-full/187112
* Leading whitespaces reformatted by vscode
* Localizable disk space progress bar title
* Kudos to https://forum.openwrt.org/u/psherman for coming up with final design

Signed-off-by: Stan Grishin <stangri@melmac.ca>