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* Remove outdated comment in common.js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

config/settings.yml

@@ -24,6 +24,7 @@ defaults: &defaults
   auto_play_gif: false
   system_font_ui: false
   noindex: false
+  theme: 'default'
   notification_emails:
     follow: false
     reblog: false