Webpack should use postcss so that SCSS doesn't need to manually include prefixed styling rules

Description

Currently when writing new SCSS we need to add in prefixed versions of some rules to support older browsers. This isn't something that should be left up to the developer to do.

Proposal

When compiling the SCSS down to CSS webpack should be able to make use of postcss to prefix rules automatically where needed. This will reduce the amount of unneeded SCSS written and will mean that rules don't need to be manually removed as newer browsers remove the need for the prefixed versions of the rules.

Links / references

PostCSS: https://github.com/postcss/postcss
PostCSS-Loader: https://github.com/postcss/postcss-loader