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

Gemfile

@@ -153,7 +153,6 @@ end
 
 group :production do
   gem 'lograge', '~> 0.11'
-  gem 'redis-rails', '~> 5.0'
 end
 
 gem 'concurrent-ruby', require: false