Add Snap packaging

Please consider allowing the Snap packaging to be included in your repository.

If you accept the Snap packaging, the Ubuntu Desktop team can easily automate nightly builds from the stable and master branches. Snap includes multiple channels. Nightly builds from the master branch can go to the edge channel and builds from the stable branch can go to the candidate channel so they can be verified to work before being manually pushed to the stable channel.

If you have any issues with the Snap packaging, please contact the Ubuntu Desktop team. If you're interesting in helping maintain the Snap version, please let us know too.