Merge tag 'v1.6.0rc4' into sync/upstream-1.6.0rc4

Conflicts:
      app/javascript/mastodon/features/getting_started/index.js
      app/javascript/packs/public.js
      app/javascript/styles/components.scss

config/initializers/sidekiq.rb

@@ -9,6 +9,9 @@ end
 
 Sidekiq.configure_server do |config|
   config.redis = redis_params
+  config.client_middleware do |chain|
+    chain.add Mastodon::UniqueRetryJobMiddleware
+  end
 end
 
 Sidekiq.configure_client do |config|