Use Hugo's resources.Minify on SVG icons

[tanyabouman]

- Summary

Changes:

• remove dimensions that made the icons not fit properly and move the icons into the assets folder so that Hugo can access the resources
• remove svg partial and modify social-icon partial to use the icons with minification
• remove extra icon.svg. I couldn't find out where it is used
• edit README to describe how to use icons

Fixes #213

- Description for the changelog

Use Hugo's resources.Minify on SVG icons