Add task to update statistics on students' activity
Function to calculate all the statistics and migration
Optimize statistics algorithm so it uses less ram
Bugfix in stats, increments didn't work
Reactivate the stats interface in teacheradmin...

...and adapt it to the new mongo collections with the stats.
Statistics command
Merge branch 'master' into mongo_refacts

Conflicts:
	moocng/api/resources.py
Rename stats migration to avoid collision
Complete statistics command
Activity creation stats handler
Answer creation stats handler
Answer update stats handler
Merge branch 'master' into mongo_refacts

Conflicts:
	moocng/settings/common.py
Handle P2P submissions creation events
Add P2P reviews creation handler
Merge branch 'master' into mongo_refacts
Avoid async-related problems in the stats tasks
Improve the charts, and the course blaklisting in command
Bugfix in stats calculation

Don't crash with activities without unit ids
Merge branch 'master' into mongo_refacts