Reorganize dependencies in package.json

[eyeseast]

Right now, our dependencies and devDependencies are mixed, in part because of Netlify and Svelte. Most dependencies are only needed during our build step, but putting them in devDependencies means they won't be installed when NODE_ENV=production. We should figure out a better way to organize these.

[eyeseast]

I think this is done as of #1311. Everything in dependencies will install even when NODE_ENV=production, and that's what we need to build the site. We'll use devDependencies for local development and testing.