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

- `secrets.secret_token` is now used in all places `config.secret_token` was
  - `secrets.secret_token`, when not present in `config/secrets.yml`,
    now falls back to the value of `config.secret_token`
  - when `secrets.secret_token` is set, it over-writes
    `config.secret_token` so they are the same (for backwards-compatibility)
  - Update docs to reference app.secrets in all places
    - Remove references to `config.secret_token`, `config.secret_key_base`
- Warn that missing secret_key_base is deprecated
- Add tests for secret_token, key_generator, and message_verifier
  - the legacy key generator is used with the message verifier when
    secrets.secret_key_base is blank and secret_token is set
  - app.key_generator raises when neither secrets.secret_key_base nor
    secret_token are set
  - app.env_config    raises when neither secrets.secret_key_base nor
    secret_token are set
- Add changelog

Run focused tests via
ruby -w -Itest test/application/configuration_test.rb -n '/secret_|key_/'