2 new strings for communitheme snap

[Didier Roche]

Hey translation team,

I want to bring to your attention that I opened an UIFe for adding 2 new 
strings which won't appear by default. Those won't appear anywhere by 
default before you install the snap.

The goal is to enable the communitheme team to deliver there awesome WIP 
as a snap easy on bionic. More information on 
https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/1760049.

Cheers,
Didier