Description

After merging #7464 we started using two different pipelines to manage assets: sprockets (the old way) and webpacker (the new way).

This upgrade was very necessary, but it also made impossible upgrading a production application. For example, at the moment you cannot create a review app to test a specific feature.

After some work I succeed, and I have this application working: https://decidim-staging-pr-341.herokuapp.com/

Let me explain a bit which steps I followed to make it work:

1. First I opened a PR in decidim to add some things that are needed. You can see it in here and there is a more extended explanation there. Briefly, we need to compile webpacker assets when we precompile assets in a production environment.
2. In the application there was more work involved, but it is related that we are using Heroku for deployment. After adding the webpacker initializer file (see chore: setup webpacker codegram/decidim-staging#341) and upgrading decidim I needed to make sure we weren't including the node_modules in the application result (they call it slug).

In conclusion, production applications are possible after merging #7915 but you may need to take some extra steps depending on your environment.

The webpacker migration is still in development, you can see more progress in here:
#7733

Thanks again to @ferblape to make this possible because I really think we should move forward and create a more modern asset pipeline in this application.