Ticket 4912 (Users List messup in UsersManager settings UI)

Comparing master branch to stable 2.1.0 version, I've discovered a new column in the table id Users called "Last Seen".

Adduser javascript weren't updated so I've integrated an empty cell that would format correctly the "addUser row".

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)