Update Updater UX to support System Installs on Windows

Mullvad Browser installed to %PROGRAMFILES% would need elevation to perform incremental updates. We have a few options here:

• Update the UX to notify users of when an update is available 😞
  • this pathway is kind of needed for various Linux packages (flatpak, deb, rbm, etc)
• Enable the update pathway the default Firefox uses to get around this problem
  • Probably involves running some system service which can with elevated privileges; Mullvad Browser doesn't care about disk leaks like Tor Browser does, so maybe this is fine
• Somehow hand waving enable the browser to self-elevate and update

/cc @pierov @ma1 @ruihildt @donuts

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