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).

app/models/ip_address.rb

@@ -0,0 +1,28 @@
+class IpAddress < ApplicationRecord
+  belongs_to :model, polymorphic: true
+  belongs_to :user
+  attribute :ip_addr, IpAddressType.new
+
+  def self.model_types
+    %w[Post User Comment Dmail ArtistVersion ArtistCommentaryVersion NoteVersion WikiPageVersion]
+  end
+
+  def self.search(params)
+    q = super
+    q = q.where.not(model_type: "Dmail") unless CurrentUser.is_admin?
+    q = q.search_attributes(params, :user, :model_type, :model_id, :ip_addr)
+    q.order(created_at: :desc)
+  end
+
+  def self.group_by_ip_addr
+    group(:ip_addr).select("ip_addr, COUNT(*) AS count_all").reorder("count_all DESC, ip_addr")
+  end
+
+  def self.group_by_user
+    group(:user_id).select("user_id, COUNT(*) AS count_all").reorder("count_all DESC, user_id")
+  end
+
+  def readonly?
+    true
+  end
+end