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

config/application.rb

@@ -24,7 +24,6 @@ module Mastodon
 
     # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
     # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
-
     config.i18n.available_locales = [
       :en,
       :bg,
@@ -35,6 +34,7 @@ module Mastodon
       :fr,
       :hu,
       :ja,
+      :nl,
       :no,
       :pt,
       :ru,