Research partial upgrades without ISO

This is about releasing security updates without putting out a new ISO. Candidate update mechanisms: APT, Flatpak, Firefox’s own update mechanism). See summit-2018.git:community_days/is_it_ok_to_skip_a_release.mdwn for the initial discussion.

In order to do this we need serious funding.

Original created by @u on 15901 (Redmine)