Replace SCSS mixins with PostCSS functions

Created by: valerybugakov

Problem statement

Success criteria

All SCSS mixins are replaced with a combination of PostCSS functions and CSS variables.

Implementation details

An example of PostCSS functions we use for breakpoints can be find here: client/wildcard/src/global-styles/breakpoints.scss. DM @valerybugakov in Slack to discuss the addition of new PostCSS plugins if they are required.

Time estimate

Pull requests with ~450 lines changed should take 4 hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.
Split the work into multiple pull requests if the total diff is bigger than 450 lines of code.