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/well_known/webfinger_controller_spec.rb

@@ -25,7 +25,7 @@ describe WellKnown::WebfingerController, type: :controller do
       end
 
       it 'returns application/jrd+json' do
-        expect(response.content_type).to eq 'application/jrd+json'
+        expect(response.media_type).to eq 'application/jrd+json'
       end
 
       it 'returns links for the account' do