added comment count to statistic to calculate posts/comments ratios

fixeds tests for comment count in statistics

added changelog entry and default entry
Merge pull request #4799 from JannikStreek/feature/4778-comment_counters

Added count of comments to statistics
Put #4799 changelog to correct place
Add devise_lastseenable for user last seen tracking.
Adds a timestamp to user model which is touched when user comes back. Statistics active users counts now use these more reliable timestamps. Fixes #4734
Merge pull request #4802 from jaywink/devise_lastseenable

Add devise_lastseenable for user last seen tracking.