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).

Gemfile.lock

@@ -171,6 +171,7 @@ GEM
       multi_xml (>= 0.5.2)
     i18n (1.7.0)
       concurrent-ruby (~> 1.0)
+    ipaddress (0.8.3)
     jmespath (1.4.0)
     jquery-rails (4.3.5)
       rails-dom-testing (>= 1, < 3)
@@ -326,6 +327,9 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.8.0)
       nokogumbo (~> 2.0)
+    scenic (1.5.1)
+      activerecord (>= 4.0.0)
+      railties (>= 4.0.0)
     selenium-webdriver (3.142.6)
       childprocess (>= 0.5, < 4.0)
       rubyzip (>= 1.2.2)
@@ -435,6 +439,7 @@ DEPENDENCIES
   ffaker
   flamegraph
   httparty
+  ipaddress
   jquery-rails
   listen
   mechanize
@@ -465,6 +470,7 @@ DEPENDENCIES
   ruby-vips
   rubyzip
   sanitize
+  scenic
   selenium-webdriver
   shoulda-context
   shoulda-matchers