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/requests/host_meta_request_spec.rb

@@ -6,7 +6,7 @@ describe "The host_meta route" do
       get host_meta_url
 
       expect(response).to have_http_status(200)
-      expect(response.content_type).to eq "application/xrd+xml"
+      expect(response.media_type).to eq "application/xrd+xml"
     end
   end
 end