Report outdated packages imported from a different channel

Some Apertis packages come from a different Debian release. In order keep
them in sync, active descendant branches are analyzed to determine in which
apertis versions they landed. Then, this information is used to report on the
dashboard which apertis releases need to be updated.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>