Move to Electron 8

[heyhippari]

• Moves from NPM to Yarn in order to be consistent with the other Web projects
• Removes most warnings resulting from the upgrade from Electron 3 to Electron 8
• Refactors the app creation
• Removes the double windows nonsense
• Fixes fullscreen switching

Most of the changed lines come from the move from package-lock.json to yarn.lock.