challenges: more consistent scheme for task lists. Same for all list types, _page.lists.tasks.{OWNER_ID}. can be a challenge id, user id, etc. Update API so it doesn't require refLists when pushign / deleting (they're only necessary for rendered pages really) @yangit
challenges: use model.push on challenges & challenge tasks for now,
unshift having strange issues (see https://github.com/SLaks/racer/issues/14)