Fix hints and leaderboard errors.

[ayu-ch]

• Leaderboard cache invalidation on hints and registration.
• Free hints for admin and fix redeem error.

[sukhman-sukh]

@ayu-ch @v1bh475u , I have pushed some changes for leaderboard cache invalidation logic. Suggest me if something is still left.
Also, Right now, challenge deletion does not change any points or cache. Should we put that into purge?

[v1bh475u]

@ayu-ch @v1bh475u , I have pushed some changes for leaderboard cache invalidation logic. Suggest me if something is still left. Also, Right now, challenge deletion does not change any points or cache. Should we put that into purge?

Yes. Whenever a challenge is gone, all the points related to it also vanish!