Add gulp-autoprefixer to our Gulp styles pipeline and remove _prefix() sass mixin

[Smona]

gulp-autoprefixer transparently prefixes the outputted css for maximum browser compatibility. The forty theme used a custom mixin that we no longer want to support.

• [x] Add gulp-autoprefixer to our gulpfile style task
• [x] Remove all instances of the forty theme prefix mixin

[NathanAB]

Unless I'm missing something, this already seems to be the case: https://github.com/politicalrev/political-revolution.com/blob/master/web/app/themes/thepoliticalsage/gulpfile.js#L102

[Smona]

Oh, neat! Don't know how I missed that 😂

[Smona]

Completed in 091e05e