Add new IP address search page.

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).

db/migrate/20191111004329_create_ip_addresses.rb

@@ -0,0 +1,5 @@
+class CreateIpAddresses < ActiveRecord::Migration[6.0]
+  def change
+    create_view :ip_addresses
+  end
+end