Create user directory

## Summary
Automatically create the user folder upon user registration. The user folder's name is generated by hashing the user's `api_token`.

### Changes
1. Created a module `Modules::Directory` in `lib/modules/directory` for dealing with directory creation process.
2. Created a config file `config/config.yml` to store the parent of all user folders and automatically load it as `APP_CONFIG` variable
3. User folder is no longer be a parameter in the request as it is generated by server by hashing the user's `api_token`
we can just write it in this way:
Dir.mkdir "#{Rails.root} #{build_users_directory} #{dir_name}"