Filter on allowed user language preferences (#2361)

* Naive approached to timeline filtering

* Convert allowed_languages into a db column

* Allow users to choose languages to see statuses in

* Style list items as two columns

* Add a hint to explain language filtering preference

db/migrate/20170423005413_add_allowed_languages_to_user.rb

@@ -0,0 +1,6 @@
+class AddAllowedLanguagesToUser < ActiveRecord::Migration[5.0]
+  def change
+    add_column :users, :allowed_languages, :string, array: true, default: [], null: false
+    add_index :users, :allowed_languages, using: :gin
+  end
+end