Make custom_sources_list into a class paramter

and thus remove the last global variable.
Drop Debian Etch and Lenny compatibility.

Both have been unsupported for ages.
Remove Lenny-related comment that was made obsolete a year ago by commit 822950.
Adapt documentation to take into account we haven't been supporting Lenny for more than a year.
add jessie preferences file
Merge branch 'master' of buse.riseup.net:shared-apt
add apticron_jessie symlink, same version in wheezy and sid
add apt listchanges template symlink for jessie - same as the others
Change default 50unattended-upgrades on Debian

* Use squeeze-lts origin on squeeze
* All other releases use newer Origins-Pattern format
* Drop support for lenny
make it possible to opt out of MailOnlyOnError

This combines all files into one template. It should make maintenance
easier.
Parametrize unattended-upgrades mail recipient