Change language opt-out to language opt-in (#7823)

* Switch filtered_languages to chosen_languages

* Adjust interface

* Remove unused translations

db/migrate/20180616192031_add_chosen_languages_to_users.rb

@@ -0,0 +1,5 @@
+class AddChosenLanguagesToUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :chosen_languages, :string, array: true, null: true, default: nil
+  end
+end