Add Dutch translation (nl) (#1682)

Add Dutch translation (nl) (#1682)

* Dutch language files for Ruby code

 * Created RoR translation ymls:
   * config/locales/devise.nl.yml
   * config/locales/doorkeeper.nl.yml
   * config/locales/nl.yml
   * config/locales/simple_form.nl.yml

 * Modified RoR config and helper
   * app/helpers/settings_helper.rb
   * config/application.rb

* Dutch language javascript locale

 * Created javascript locale files:
   * app/assets/javascripts/components/locales/index.jsx
   * app/assets/javascripts/components/locales/nl.jsx

 * Reference the newly created locale files:
   * app/assets/javascripts/components/containers/mastodon.jsx

* Fix syntax error in locale file (nl)

* Fix missing translate in js locale (nl)

* Convert all ruby Dutch locale (nl) file to utf8

Fix yml conversion issues.

* Fix duplicated key in devise.nl.yml

* Fix indentation error in doorkeeper.nl.yml