Upgrade everything to match Electron v10 and etc

[Rinkaa]

I'm quite new to the JS world and I just found this precious boilerplate as my first attempt to developing Electron project. However when I npm install, npm told me many of these dependencies were deprecated, and the boilerplate was partly broken when I tried to run the demo using Electron v10.1.0 . gulp serve was not working as expected, and the js file built had errors so the page could not be shown correctly.
After some digging about Electron related solutions, I tried to rebuild this boilerplate based on this repo, with all dependencies and configurations more up-to-date.

Major changes:

• upgrade Electron to v10
• upgrade React to v16, app/js/*.js slightly modified
• upgrade Babel to v7
• upgrade gulp to v4, and gulpfile.js correspondingly
• upgrade eslint to v7, and .eslintrc.json correspondingly
• replace electron-connect with Electronmon for hot reloading
• replace electron-prebuilt with electron-packager for packaging
• add gulp-sass-lint for linting *.sass (because lint everything in readme.md)
• a new moving pic based on the new code files