Use rails secrets

#### Move third party credentials to secrets.yml as recommended in [rails guides](http://edgeguides.rubyonrails.org/4_1_release_notes.html#config/secrets.yml). 

secrets.yml - stores key value pairs, where value should be taken from `ENV` and be env specific. We suppose to share through secrets only secret data: third party credentials(rollbar, sendgrid, facebook), encryption keys etc

Other configs, like a mail_safe whitelist should not be stored in the secrets.yml and should be taken from `ENV` directly or through [custom configurations](https://github.com/fs/rails-base/issues/118)
I wanted to extend Kernel before requiring `config/initializers`, to have an access for `app_config` and `app_secrets` from tantalizers.