Move electron build script to /tools/

Also enables bash strict mode flags for the build script

tools/package-electron.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -euxo pipefail
+
+# Clear out any files remaining from old builds
+rm -rf .package
+
+mkdir -p .package/dist/ || true
+
+cp index.html .package
+cp favicon.ico .package
+cp -r electron/* .package
+cp -r dist .package
+
+# Install electron sub-dependencies
+cd electron
+npm install
+cd ..
+
+BUILD_PLATFORM="${1:-"all"}"
+# And finally build the app.
+npm run electron:packager-$BUILD_PLATFORM