Do not use rails_config

[We are using it](https://github.com/monterail/guidelines/blob/master/gems.md#general) for a while and we know that it is very unstable (most of last versions doesn't work at all...). For sensitive config use `figaro` instead. I'm not sure what should we use for global settings and which could be commit into repo.

At least we can put such values into `application.yml.example` files.

/cc @chytreg @teamon @Ostrzy @szajbus @sheerun @jcieslar