Add calculated fields to User model
Add calculated fields to User model

Add User#completed_count to provide number of completed sessions for
a tutor.
Add User#recent_count to provide number of completed sessions for a
tutor in last 45 days.
Add User.leaderboard to provide array of top ten tutors over past 45
days based on number of completed sessions.

theresaboard/theres-a-board,theresaboard/theres-a-board,theresaboard/theres-a-board