which it turns out is by far the lesser of two evils.
* Auto-update works with a proxy
* The update process is reasonably atomic & faster, rather than
running the uninstaller then the installer, leaving you with a
broken install if you shut down your machine at the wrong time
* Gets the update URL the same way as on mac, rather than baking
it into the app at build time from package.json. We don't want
it in package.json because only our builds want our update URL.
Amalgamate the electron build packaging into one script.
Use update_base_url so we can compute the actual URL in the
script for windows (because we need to put it in the build) and
at runtime for mac os.
David Baker