Luci opkg/packages: Show package size in list of available packages

Add package *.ipk size information to package listing in Luci,
as opkg was today extended to support listing also the size information.
Visible fields are now: name, version, size, description

That will help users considering installation of a certain package
to assess its size impact on flash.

Note: Opkg data includes the size of the .ipk file, not the expanded size.

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