* 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
* Add tests for some cachable responses
This only covers responses that we should have managed to make cachable
so far. It's not the case of all responses that should be cachable in
the end.
* Fix RSS feeds not being cachable
