SVG sprite method change

[evanmwillhite]

Note: This and the matching PR for Emulsify Gulp must be merged/released simultaneously

Switches SVG sprite method to use symbol (from background CSS).

To Test:

• Clone a fresh copy of Emulsify and set your Emulsify Gulp devDependency in package.json to fourkitchens/emulsify-gulp#SVG-method-switch
• Install as usual and yarn start
• Verify the Icons section looks OK as well as any components using an icon
• Verify any leftover code has been removed from the background CSS approach
• Follow the instructions in components/_patterns/01-atoms/04-images/icons/icons.md to add new icons and test a variety of ways of using them.
• Verify all works accordingly.