Generate SCSS into individual CSS files within components_combined.

An additional script splitcss.js was added to the npm run dist step. It runs after the existing webpack, and will build the SCSS files into their respective component folders, and adds a base.css to the components_combined/base folder for the core required styles for every component.

Some notes:

• As this SCSS build is run independently of the webpack build, no loaders were used in the process - only features available in the npm sass package. This means any ~ imports have been replaced with a relative path to the file. (See breakpoint and reset).
• Some mixins have been defined within a component scss file. This means if the mixin needs to be used elsewhere, it will double-import the selectors that also live with the mixin. For this reason, I have split out the mixins into the mixin folder, where we can be safe importing it without adding unwanted selectors. See background and button.