`secret_token` is now saved in `Rails.application.secrets.secret_token`

- `secrets.secret_token` falls back to the value of `config.secret_token` when it is not present in `config/secrets.yml`
  - thus we no longer raise when this happens
- when `secrets.secret_token` is set, it over-writes
  `config.secret_token` for backwards-compatibility

Some of the guides may require more changes to remove references
to setting secrets in a config and replace with setting them in
config/secrets.yml

This commit is against master. The commit against 4-1-stable is
https://github.com/rails/rails/commit/dd018ec38162d8e5a13915b0d64689bd7edcc26d