Auto-update & build process with NSIS for windows

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.

README.md

@@ -107,6 +107,9 @@ You can configure the app by copying `vector/config.sample.json` to
    valid location on this network. This is used as a hint to the user to indicate
    when a valid location has been entered so it's not necessary for this to be
    exactly correct. Optional.
+1. `update_base_url` (electron app only): HTTPS URL to a web server to download
+   updates from. This should be the path to the directory containing `install`
+   and `update`.
 
 Running as a Desktop app
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