Upgrade to Next.js 9.4

[kmjennison]

A number of update affect us and allow us to strip out custom code:
https://nextjs.org/blog/next-9-4

[kmjennison]

Note that we may want to rearrange our .env files:
https://nextjs.org/docs/basic-features/environment-variables

[kmjennison]

To do:

• Update to Next.js 9.4
• Use built-in basePath
• Use built-in Next.js rewrites and redirects, removing Vercel rewrites and redirects -- wait until stable in later version of Next
• Use built-in fetch
• Use built-in absolute imports
• Use built-in .env file support and possibly rename public env vars
• Refactor auth logic and switch to getServerSideProps (see Add example: Firebase authentication + SSR kmjennison/next.js#1) -- moved to separate issue Refactor Next.js/Firebase auth logic and switch to getServerSideProps #79