Add a new IP address search page at /ip_addresses. Replaces the old
search page at /moderator/ip_addrs.
On user profile pages, show the user's last known IP to mods. Also add
search links for finding other IPs or accounts associated with the user.
IP address search uses a big UNION ALL statement to merge IP addresses
across various tables into a single view. This makes searching easier,
but is known to timeout in certain cases.
Fixes#4207 (the new IP search page supports searching by subnet).
