Electron Bug Fixes and Miscellaneous Changes

[k9ert]

• migrate the downloadpage generation script to this repo and updated the release-guide.md (+ autosave changes also in self-signed-certificates.md))
• set the target for the build to universalwhich is unfortunately mostly undocumented (issue).
• Some missing or misguided variable declarations in the refactored electron app which caused:
  • specter-desktop didn't terminate properly
  • Seeing transaction details results in a crashing app
  • Saving preferences was not possible

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[moneymanolis]

Built the electron app and everything seems to work, nicely done!