Remove Gulp as a dependency

I love Gulp, but I love less external dependencies more. I initially used it for the redesign to get things up and running quickly.

The tasks we're using Gulp for are relatively straightforward, namely asset minification, concatenation, post-processing, and Browsersync. Because of this, there is an opportunity to do a lot of this via node scripts. Big-picture, I'd also like to lean more on Eleventy's capabilities to do the same—we're already using it for things like addPassthroughCopy.

To remove Gulp, we'd need to:

• Create node scripts for each Gulp task, and ensure feature parity.
• See what tasks can be used in Eleventy directly.
• Update npm run a11yproject to make sure it works.
• Update Netlify to reflect these changes.
• Remove references to Gulp in our documentation.
• Do a little happy dance.