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

lib/mastodon/redis_config.rb

@@ -27,6 +27,8 @@ namespace       = ENV.fetch('REDIS_NAMESPACE', nil)
 cache_namespace = namespace ? namespace + '_cache' : 'cache'
 
 REDIS_CACHE_PARAMS = {
+  driver: :hiredis,
+  url: ENV['REDIS_URL'],
   expires_in: 10.minutes,
   namespace: cache_namespace,
 }.freeze