Add store_location_for helper

This is used as a complement to `stored_location_for`.

Example:

Before authorizing with Omniauth;

  store_location_for(:user, dashboard_path)
  redirect_to user_omniauth_authorize_path(:facebook)

In our Omniauth callback

  sign_in(user)
  redirect_to stored_location_for(:user) || root_path