Filter languages with opt out (#3175)

* Remove allowed_languages and add filtered_languages

* Use filtered_languages instead of allowed_languages

db/migrate/20170520145338_change_language_filter_to_opt_out.rb

@@ -0,0 +1,9 @@
+class ChangeLanguageFilterToOptOut < ActiveRecord::Migration[5.0]
+  def change
+    remove_index :users, :allowed_languages
+    remove_column :users, :allowed_languages
+
+    add_column :users, :filtered_languages, :string, array: true, default: [], null: false
+    add_index :users, :filtered_languages, using: :gin
+  end
+end