Bug#754694: "Flag as outdated" feature implementation.

[Alivema4ever <alivema4ever@gmail.com>]

I suggest to implement flag package as outdated for debian package 
tracker, especially for unstable. This will cover informational update 
for some packages, which in most of cases have their homepages moved 
or abandoned.

Archlinux packages site has a function to flag packages as outdated. 
Well, it may be target to spam attact, but some workaround exist to 
prevent that from happening, such as recaptcha challenge on flagging 
package as outdated or manual review by Debian packaging team before 
an outdated package flag is accepted.

This will make automatic package update notification helpful and more 
accurate to provide information about outdated packages to maintainer.