Implement automatic updates

[phenaproxima]

A fairly big lift, but the launcher app needs to be able to update itself, despite its essentially disposable nature. Electron's ecosystem has very robust support for doing auto-updates, but we have to get it up and running. This is table stakes.

[phenaproxima]

Assigning @mccoyweb to track this issue, where is the overarching issue where we'll pursue automatic updates.