ziafazal/api-added-ability-to-skip-leaders-list: ability to skip leaders list

@mattdrayer  @martynjames @chrisndodge added ability to skip leaders list via `skipleaders` flag in `CoursesMetricsCompletionsLeadersList`. So in case of home page we can get only user's progress and course average by hitting the api like this
`/api/courses/{course_id}/metrics/completions/leaders/?user_id={user_id}&skipleaders=true`