Added snap packaging

This adds snap packaging which will help us get automated builds publishing to the snap store. If this is accepted, I'll do the same for the gnome-3-32 branch. Our automated builds from master will be published to the edge channel and the builds from the latest stable branch will be published to the candidate channel. After what's in the candidate channel passes QA it will be published to stable.

I'm happy to maintain the snap packaging, if there are bugs filed feel free to assign them to me.