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* Remove outdated comment in common.js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

config/locales/simple_form.en.yml

@@ -13,6 +13,7 @@ en:
           one: <span class="note-counter">1</span> character left
           other: <span class="note-counter">%{count}</span> characters left
         setting_noindex: Affects your public profile and status pages
+        setting_theme: Affects how Mastodon looks when you're logged in from any device.
       imports:
         data: CSV file exported from another Mastodon instance
       sessions:
@@ -44,6 +45,7 @@ en:
         setting_noindex: Opt-out of search engine indexing
         setting_system_font_ui: Use system's default font
         setting_unfollow_modal: Show confirmation dialog before unfollowing someone
+        setting_theme: Site theme 
         severity: Severity
         type: Import type
         username: Username