AoC norush extension per vhermecz

Extends private leaderboard capabilities of the adventofcode.com website by tracking the first time a user opens a day's challenge.
Uses a backend to store the first-opened times for each user.
Once you first visit a day's task, the browser extension sends this timestamp to the backend. It also scans your page visit history of AoC problems/days, so it works for past events as well.
When you open a private leaderboard, NoRush queries first-opened times for all members from the backend and computes the scores based on that.