Limit Next.js delegation to GET/HEAD requests

[victorlin]

Without this, some invalid POST requests would time out or fail with a 500 server error:

curl -i -X POST \
  -H "Content-Type: multipart/form-data" \
  -H "Accept: text/html" \
  http://localhost:5000/

These now respond with 404 not found.

Slack thread

Checklist

• Checks pass

• Tested locally

   $ curl -i -X POST \
     -H "Content-Type: multipart/form-data" \
     -H "Accept: text/html" \
     https://next.nextstrain.org/
   HTTP/2 503
   
   $ curl -i -X POST \
     -H "Content-Type: multipart/form-data" \
     -H "Accept: text/html" \
     https://nextstrain-s-victorlin--3cy38x.herokuapp.com/
   HTTP/1.1 404 Not Found

• Check if changes affect the resource index JSON revision