Prepare Mastodon for Rails 6 (#15911)

* Fix misuse of foreign_type

* Fix use of removed "add_template_helper"

* Use response.media_type instead of response.content_type in tests

* Fix CSV export controller test on Rails 6

Rails 6 sets a "filename*" field in the Content-Disposition header to
explicitly encode the filename as UTF-8.

This changes checks the first part of the Content-Disposition header so
it matches in both Rails 5 and Rails 6.

* Fix emoji formatting with Rails 6

* Make emoji output more idiomatic and robust

* Switch from redis-rails gem to built-in Rails redis cache storage

config/initializers/sidekiq.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 namespace    = ENV.fetch('REDIS_NAMESPACE') { nil }
-redis_params = { url: ENV['REDIS_URL'] }
+redis_params = { url: ENV['REDIS_URL'], driver: :hiredis }
 
 if namespace
   redis_params[:namespace] = namespace