norm_ip: Use /23 subnet for IPv4

Apparently there are networks where a single visitor is assigned random
addresses in the /23 range for each request. This should fix the
login/registration form on such networks, and makes the login throttling
more robust (and easier to trigger for innocent people, but judging from
monitoring the throttle table, failed logins arent that common).

I wonder if /23 is enough, but we'll see.
Change normalization factor in popularity ranking calculation

Suggested by Hinoe, quoting his reasoning:

  In popularity rankings, change the normalization from
  "sqrt(LowerVoteCount)" == "LowerVoteCount^0.5" to something that grows
  somewhat more slowly.
  Details: Natural logarithm itself (ln(LowerVoteCount+1)) is too slow;
  at the current VN count (15403), it returns 9.64; however, sqrt(15402)
  is just above 124.1, which I feel is already too high. After
  experimenting with the exponents a bit, I decided that the best point
  likely lies between 0.3, which returns a bit above 18.0, and 0.4,
  which returns a bit above 47.3. Thus, I suggest that the new function
  be LowerVoteCount^0.36788; the exponent is a 5-digit approximation of
  e^-1, just because it's a nice number in the specified area and works
  well, returning circa 34.7.
Multi: Update maintenance timings + disable usercache

The usercache maintenance cron is causing significant downtime each
month, so I've disabled it for now.