* 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

* Code quality pass

* Typofix

* Update applications_controller_spec.rb

* Update applications_controller_spec.rb

* Spec output of Settings::Exports::BlockedAccountsController

* Spec output of Settings::Exports::FollowingAccountsController

* Spec output of Settings::Exports::MutedAccountsController

* Spec Settings::Exports::BaseController

This commit removes duplicate specs in classes inheriting
Settings::Exports::BaseController as well.