Remove node_modules and assets from Heroku's slug

[fjsj]

Depends on #561

After all files are on S3, it's safe to remove them from Heroku's slug. Use this: https://github.com/vintasoftware/heroku-buildpack-post-build-clean

[hugobessa]

We're not supporting Heroku anymore so this has lost its relevance

[fjsj]

how the container is generated on Render, though? does it have any ignore paths?