DFS contest slate seasons and schedule manifest

Returns dynamic available seasons, weeks, and date bounds for DFS contest slates. NFL weeks are regular-season schedule windows in US Eastern time: week N runs from the day after week N-1's last game to week N's last game day. The windows are disjoint, so each slate belongs to exactly one week (the week of its lock date); preseason and postseason slates belong to none. To list one week's slates, pass the week's start_date and end_date to /api/v1/dfs/slates as date__between=start_date,end_date.