Auto-Prefix CSS in GitLab

One of the reasons we want to do https://gitlab.com/gitlab-org/gitlab-ce/issues/29346 is that it enables us to do auto-prefixing.

Auto-prefixing (https://github.com/postcss/autoprefixer) is a technique to make CSS compatible with older browsers, e.g. turning:

select {
  appearance: none;
}

into

select {
    -moz-appearance: none;
    -webkit-appearance: none;
}

based on data from caniuse.com.

We do not necessarily need compiling our SCSS within webpack, we can already accomplish this with https://github.com/ai/autoprefixer-rails in our Sprockets pipeline