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

app/mailers/user_mailer.rb

@@ -8,7 +8,7 @@ class UserMailer < Devise::Mailer
   helper :instance
   helper :statuses
 
-  add_template_helper RoutingHelper
+  helper RoutingHelper
 
   def confirmation_instructions(user, token, **)
     @resource = user