dashboard: rename 'rating cycle' to 'session' for display purposes
tournament: add column: 'multi_session'

to indicate if tournament has multiple sessions
tournament: rename field 'current_rating_cycle' to 'current_session'
begin a TODO file
schema: rename contest.rating_cycle field to contest.session_num
ratings: rename 'Week' to 'Session' in the output,

and hide the '0th' week
ratings: only include players who participated in ratings calculations
make 'session' field contingent on multi_session being enabled
ratings: ability to commit ratings to new 'player_ratings' table
contest_participants: show player's rating