Add number of solved archive challenges to whois output

[RobinJadoul]

See also cryptohack/issues-tracker#3

[hyperreality]

Have added /api/site-stats/ (in dev currently) that returns the following:

{
  "category_challenge_count": {
    "aes": 24,
    "ctf-archive": 55,
    "diffie-hellman": 14,
    "ecc": 19,
    "general": 19,
    "hashes": 11,
    "introduction": 3,
    "maths": 23,
    "misc": 14,
    "post-quantum": 10,
    "rsa": 29,
    "web": 17,
    "zkp": 9
  },
  "challenge_non_archive_count": 192,
  "user_count": 9
}

It should be possible to move the bot to using the public API now, and I'll raise the rate limits so the bot shouldn't hit them