Add option 'adhocracy.monitor_pager_clicks'.

Will be used to (de)activate monitoring of pager clicks.
Log the list index and active sort of pager clicks

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.
Make page stats baseurl always available.
Remove dependency between on-page ping and pager click stats.
Correctly check for element existence.
Use jQuery.isEmptyObject to check whether properties are set.
Add comment to cookie deletion.
Use RegEx literal instead of a string.
Merge pull request #401 from wawe/feature-monitor-pager-clicks

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