Maybe a more specific view config for embed

I'm thinking that somehow the problem might be that the html
view is taking precedence, so make them both specify the accept
headers. This way, the embed is more specific than the frame view
in routing precedence.
Auto-detect Heroku / Dokku in gunicorn.conf.py

This sets up the forwarded_allow_ips configuration which is needed
for the app to trust X-FORWARDED-* headers.