Code review comment for lp://staging/~laney/unity8-desktop-session/tweaks

I'm revoking my approval for this because the lightdm files should probably be installed into /etc/lightdm/lightdm.conf.d (conffiles can cause problems on package removal). That also allows the contents of 20-unity8-mir.conf to be merged into 55-unity8-mir.conf to avoid possible confusion from having two different conf files for the same thing with different contents.