Create app.json for Heroku
Create app.json for Heroku

Created from an existing review app's https://dashboard.heroku.com/apps/auspice-docs-fixes-axgtobmfios/app-json

- Removed review-app-specific HEROKU_* vars
- Set non-secrets HOST and NPM_CONFIG_PRODUCTION values directly in the file

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice