Fix Internet Explorer support by adding polyfills and better CSS prefixes.

Quizzard used to support Internet Explorer >= 10 without problems, but due to the removal of polyfills with the move to Webpack in #20 (closed) and the requirement for new polyfills because of the front-end libraries update in #17 (closed), Internet Explorer couldn't load Quizzard anymore.

This MR ensures Quizzard works again on Internet Explorer. To be more specific, the following changes have been made:

• babel-polyfill is added to the package.json and included in nashorn.jsx to support React 16 and some other libraries.
• bootstrap.native polyfill is added to nashorn.jsx to support the Bootstrap functions (such as modals) in Internet Explorer.
• cssnano and autoprefixer are updated and .browserslist.rc is added to tell these tools which browsers Quizzard actually supports. Using npm run browserslist it is possible to get a list of supported browsers for Quizzard at any time.
• Because autoprefixer is now in charge of generating CSS prefixes, the CSS prefixes have been removed from our less files. This cleans up our CSS.
• The Quiz Editor slide tools have been updated since event.originalTarget does not work in Internet Explorer, even with polyfills in place.

This merge request is a result of the efforts in order to fix issue #33 (closed).