Make 'subnet' feature of $wgRateLimits work with IPv6

- 'subnet' will aggregate limits for a /64 subnet on IPv6
- Updated DefaultSettings.php to mention this
- Only call WebRequest::getIP() when it will really be used

Change-Id: Ia96800df5fb498a79e2c0527baee2392cd4623c7
Merge "Make 'subnet' feature of $wgRateLimits work with IPv6"