Replace deprecated autoprefixer-rails

[erbridge]

autoprefixer-rails is deprecated and no longer maintained. We should replace it with normal autoprefixer via PostCSS or similar.

[leeky]

Looks like they reversed the decision. Is it therefore ok to close this issue, @erbridge ?

[erbridge]

My understanding of the change is that the deprecation reversal fixes this for sprockets Rails, but not for webpacker, so I think this is something we probably need to do regardless.

[CristinaRO]

@erbridge How can we tell whether this is the case? I don't see anything in their readme that would make it obvious whether it works with webpacker or not.

[erbridge]

Easiest way is probably to see if CSS is still prefixed when we expect it to be.