Optional Now Playing filtered by user option

With web player and external clients, the session id often changes and Playing Now can return several songs for the same user. Depending how you use Ampache, this came frequently and can become real noise on the home page.
This pull request add a new interface option to only return the last played song per user.

The db update is update_360016 because update_360015 is alrady used by the other pending pull request #69.