Add instagram background implementation and generalize application for other identity providers

spec/acceptance_steps/global_user_steps.rb

@@ -29,7 +29,7 @@ step "the user should be redirected to the archived list path" do
 end
 
 step 'there is another user' do
-  @other_user ||= create :user
+  @other_user ||= create :user, :other_auth
 end
 
 step "the other user clicks the join table button" do