Adding ppa does not work properly on downstream distros based on Ubuntu

When adding a PPA using the "sudo add-apt-repository ppa:ppa-name" command, the distribution codename specified in /etc/lsb-release is used when adding the entry to /etc/apt/sources.list. Unfortunately, this has the side effect of causing the add PPA functionality to not work properly on downstream distros who ship with their own codename -- the user must manually change the incorrect distribution entry to the relevant Ubuntu one and refresh the package repos before using the PPA.

Downstream distros wishing to use their own repos alongside the Ubuntu ones must provide a unique distribution codename in their mirrors file in /usr/share/python-apt/templates, otherwise software-properties-gtk thinks the downstream distro's repository is a mirror of the Ubuntu one, and forces the user choose between them.

A better solution that would solve both needs would be add a check to the add PPA process:

When adding a PPA, check the current distribution codename against a set of known official Ubuntu codenames. If they match, continue as before. If they do not match, default to using the Ubuntu codename current for the installed version of software-properties package.