Quiz Editor flexbox lay-out uses browser specific prefixes

Once !158 (merged) got merged to dev, we introduced browser specific prefixes in our CSS. These browser specific prefixes should not be here anymore... We now use autoprefixer to take care of this for us, automatically creating the prefixes that are still needed for our supported browser set (based on our .browserslistrc). This overcomes having to think about browser-specific prefixes as well as checking when prefixes are needed.

In an earlier merge request, !164 (merged), autoprefixer was introduced. This branch was never merged back into MR !158 (merged), which now causes us to still have browser specific prefixes.

Although this is not high priority, it should be fixed to have a consistent code base. Also, it will prevent us from problems in the future.