Implement icon index strategy

[alkamin]

Overview

This PR streamlines the icon generation process and avoids having two copies of the theme around. Instead, this method generates an icon index file which gets imported into the default theme.

Checklist

• [NA] Relevant documentation pages have been created or updated
• Description of PR is in an appropriate section of the changelog and grouped with similar changes if possible

Testing Instructions

• Check out the icons and see that they work as they did before

Closes #108

[designmatty]

I went in and made a few adjustments to this PR for general maintenance (see commit message). I'm going to ask that you review to double check things are still as intended. Everything worked as expected from my view point.