Use the default update-electron-app for triggering automatic updates

[jkuester]

Currently we are using the electron-simple-updater library to automatically pull updates off of GitHub releases. It would be nice to switch to the default electron library instead: https://github.com/electron/update-electron-app

At the same time, it would be good to also enhance the update workflow so that the user is notified when an update to the app begins to download (in addition to the dialog that pops at the end of the download to restart the app).

[jkuester]

So, as of #16 we have a decent system of auto-updating on Windows and also at least automatically notifying OSX/Linux users that there is a new version.

Before moving ahead on this issue we would need to determine if we could at least get similar functionality for OSX/Linux where the user is notified automatically that there is a new version available. (At this point update-electron-app does not support actually auto-updating the app, similar to electron-simple-updater [though ESU theoretically can auto-update for certain linux package types...]. The UEA also does support auto-updating OSX apps without having a signed executable, again similar to what we have with ESU.)

So, the TLDR here is that switching to use update-electron-app is still on the table, but we need to make sure we have at least cross-platform feature parity before we do it.