Configure DM Sans with gatsby-plugin-web-font-loader

[deepsheth]

Currently DM Sans is being used as a dependency with the typeface-dm-sans npm package. While this provides significant performance enhancements, because the font gets loaded in as part of the webpack bundle, it does make it harder for consumers of this theme to override the default font.

By removing the typeface-dm-sans dependency, and adding DM Sans with gatsby-plugin-web-font-loader instead, we can make it easier for consumers of this theme to override.