FreeCodeCamp: FCC Leaderboard in React

User stories:

1. I can see a table of the freeCodeCamp campers who've earned the most brownie points in the past 30 days.
2. I can see how many brownie points they've earned in the past 30 days, and how many they've earned total.
3. I can toggle between sorting the list by how many brownie points they've earned in the past 30 days and by how many brownie points they've earned total.