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

spec/controllers/concerns/export_controller_concern_spec.rb

@@ -22,8 +22,8 @@ describe ApplicationController, type: :controller do
       get :index, format: :csv
 
       expect(response).to have_http_status(200)
-      expect(response.content_type).to eq 'text/csv'
-      expect(response.headers['Content-Disposition']).to eq 'attachment; filename="anonymous.csv"'
+      expect(response.media_type).to eq 'text/csv'
+      expect(response.headers['Content-Disposition']).to start_with 'attachment; filename="anonymous.csv"'
       expect(response.body).to eq user.account.username
     end