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

config/environments/development.rb

@@ -17,7 +17,7 @@ Rails.application.configure do
   if Rails.root.join('tmp/caching-dev.txt').exist?
     config.action_controller.perform_caching = true
 
-    config.cache_store = :redis_store, ENV['REDIS_URL'], REDIS_CACHE_PARAMS
+    config.cache_store = :redis_cache_store, REDIS_CACHE_PARAMS
 
     config.public_file_server.headers = {
       'Cache-Control' => "public, max-age=#{2.days.to_i}",

config/environments/production.rb

@@ -52,7 +52,7 @@ Rails.application.configure do
   config.log_tags = [:request_id]
 
   # Use a different cache store in production.
-  config.cache_store = :redis_store, ENV['CACHE_REDIS_URL'], REDIS_CACHE_PARAMS
+  config.cache_store = :redis_cache_store, REDIS_CACHE_PARAMS
 
   # Ignore bad email addresses and do not raise email delivery errors.
   # Set this to true and configure the email server for immediate delivery to raise delivery errors.