Add webpack to build stylesheets instead of Middleman

Why is this change being made?

This MR moves the task of process .scss file from Middleman to a Webpack external asset pipeline.

Basically, we are doing a straight refactoring to remove the dependency on middleman for moving/minifying the css files and replacing it with Webpack.

We are taking a iterative approach on this and implementing Webpack for stylesheets first and then adding javascripts to the webpack build later.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in [#mr-buddies][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

Related #5323 (closed)