Add support for multiple themes (#4959)

* Add support for selecting a theme

* Fix codeclimate issues

* Look up site default style if current user is not available due to e.g. not being logged in

* Remove outdated comment in common.js

* Address requested changes in themes PR

* Fix codeclimate issues

* Explicitly check current_account in application controller and only check theme availability if non-nil

* codeclimate

* explicit precedence with &&

* Fix code style in application_controller according to @nightpool's suggestion, use default style in embedded.html.haml

* codeclimate: indentation + return

app/views/settings/preferences/show.html.haml

@@ -5,6 +5,8 @@
   = render 'shared/error_messages', object: current_user
 
   .fields-group
+    = f.input :setting_theme, collection: Themes.instance.names, label_method: lambda { |theme| safe_join([I18n.t("themes.#{theme}", default: theme)])}, wrapper: :with_label, include_blank: false
+
     = f.input :locale,
       collection: I18n.available_locales,
       wrapper: :with_label,