Rewrite Advent of Code leaderboard logic

I've rewritten the Advent of Code leaderboard logic. Unfortunately,
nearly all of the changes made are interrelated, meaning that they've
ended up in the same commit.

To add a bit of structure to the extension, I've chosen for a subpackage
structure instead of a single file structure.

The biggest changes:

- Whether or not you get a join code for the staff leaderboard will now
  be determined by looking for the Helpers-role.

- The Python Discord Leaderboard now includes all boards, including the
  staff leaderboard. This is one event.

- Redis is now used to set a cache expiry period. This means that our
  code does not have to check for cache staleness; Redis will do that
  for us.

- The period "fetching" task has been removed. We now fetch solely when
  the data is needed to prevent putting unnecessary stress on the Advent
  of Code website.

- The option to display the Global Leaderboard within Discord has been
  removed. Rather, we now link to the website. This simplified the code
  for now, although we could add it back later.

- An additional command, `.aoc refresh`, has been added to allow Admins
  and the Events Lead to force the cache to be invalidated. This should
  be done sparingly to not overburden the AoC website.

- I've also made sure that the daily notification task actually pings
  the notification role by setting the `allowed_mentions` kwarg.

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