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* Remove outdated comment in common.js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/locales/en.yml

@@ -461,6 +461,8 @@ en:
     settings: Settings
     two_factor_authentication: Two-factor Authentication
     your_apps: Your applications
+  themes:
+    default: Mastodon
   statuses:
     open_in_web: Open in web
     over_character_limit: character limit of %{max} exceeded