Log the list index and active sort for clicked proposal links, JavaScript method

When a user clicks a proposal the list index and active sort are logged
into a cookie. On the next page load, this cookie is read and the data
 is send to the stats controller. The option
'adhocracy.monitor_pager_clicks' must be set to True to enable the
 functionality.
    
 This is needed for hhucn/adhocracy.hhu_theme#119.