Run Sparkle updater in background to make macOS app work offline.

[AlexCheema]

Motivation

Currently the macOS app requires internet to start, since the sparkle update runs on start in the foreground.

Changes

Run sparkle updater in background.

Why It Works

Sparkle updater no longer runs in the foreground. It now runs in the background so if it fails it will fail gracefully.

Test Plan

Manual Testing

We need to make a new build of the app and ensure it starts without internet.

[JakeHillion]

I tested both EXO-latest.dmg and the EXO-1.0.61-alpha.0.dmg and both start fine on a Mac with no Internet connection. Will leave that comment on the issue too, but currently I can't reproduce the issue this is solving.

[AlexCheema]

This doesn't seem to fix anything.