Make assignment to issues opt-in at the packager level (via bug buddy) rather than at the packages level

The idea is to have the same logic currently used to manage assignment to "unconfirmed" issues via bug buddy, where packagers could opt-in to be assigned to "New version" issues.

In terms of implementation, it implies the following:

• The package file should contain the full list of packages present in the repo (so all of them are monitored) and should be kept up to date with newly added and dropped packages, ideally dynamically/automatically. Maybe with a query to https://archlinux.org/packages/pkgbase-maintainer within the script (this is what devtools uses for that purpose)? <-- MR opened at !3 (merged)

• A new dedicated scope should be created for "New version" issue and should be added to the issues' labels list when being opened in the script. <-- Created by Levente: scope::out-of-date

• Bug buddy should be updated to link an emoji to assignment for issues containing that scope (based on the logic used for issues with the "unconfirmed" scope). <-- Bugbuddy issue created at bugbuddy#10 (closed)