Add the possibility to collect data about the behavior of users when viewing comments

Add the possibility to collect data about the behavior of users
while looking at proposals and their comments, a prerequisite to fixing
hhucn/adhocracy#115
 - introduced a 'stats' controller to be used to collect various usage
   statistics to be used by this feature as well as future extensions
   (e.g. hhucn/adhocracy#167)
 - added an option 'adhocracy.monitor_comment_behavior' to control
   whether stats about comments should be collected
 - added background requests that are triggered iff the option is set
   and a user either scrolls or opens 2nd-level comments; for data to be
   collected, 'adhocracy.requestlog_active' must be set to True