snap as a secondary distribution method

[Fuseteam]

Auto-update and adding checksums are moved to separate issues. Closing for now.

Additional distribution might be discussed in separate issues.

Originally posted by @ShGKme in #257

as per the above comment, this is a tracking issue for talk-desktop as a snap.

a few reasons for my preference of snaps are:

• install and forget: snap are auto updated
• potential for a talk client on my preferred mobile linux distro (Ubuntu Touch)
• mulitple update channels, which allow for testing pre-releases before pushing it for all users