Do not group activities that were triggered more then 3 days ago

Currently all activities are grouped, independent from the timestamp.
This means when you edited a file a month ago and today and there was no other activity in the meantime,
there will be only 1 activity. With this patch after 3 days a new activity is triggered.

Also added a little bit of tests for the grouphelper

@karlitschek @MorrisJobke