Comparing master branch to stable 2.1.0 version, I've discovered a new column in the table id Users called "Last Seen".
Then I've discovered that foreach row you were using duped IDs for forms inputs. So I've corrected duplications (IDs must be unique), adding the data-target HTML5 attribute.
There stills a problem in the users table: the last row is duped from the previous. Some help is appreciated debugging this mess...
(Ticket Reference: http://dev.piwik.org/trac/ticket/4912)
Well, thanks to github, I've easily discovered the contribution that caused regression, I had no clue why doing a foreach using reference, then I've found that PHP "unlinks" references while being in foreach and I suppose this is caused the messup...
So this pull request is my solution to :bug: 4912.
:warning: Last Seen column stills empty for each user subscribed. Looks like It's still in development so I've untouched LastSeenTimerLogger.php object.
Not merging, see: http://dev.piwik.org/trac/ticket/4912
Thanks for the report + research.