Investigate and eventually remove rails_12factor gem

[enricostano]

The rails_12factor seems stuck at 2014 and I am not sure it's needed anymore. I guess we introduced it when we were using Heroku.

We still have a Heroku integration, but we don't use it. Look at #477 (click on Show environments at the bottom of the page).

This could be related to #480 but we need to investigate a bit more.