users: log restricted signups to NewRelic.

When a new user creates an account and their account is automatically
restricted, log the reason why to NewRelic.

app/logical/user_verifier.rb

@@ -26,6 +26,10 @@ class UserVerifier
     end
   end
 
+  def log!
+    DanbooruLogger.add_attributes("user_verifier", to_h)
+  end
+
   private
 
   def ip_address
@@ -55,5 +59,9 @@ class UserVerifier
     IpLookup.new(ip_address).is_proxy?
   end
 
+  def to_h
+    { is_ip_banned: is_ip_banned?, is_logged_in: is_logged_in?, is_recent_signup: is_recent_signup?, is_proxy: is_proxy? }
+  end
+
   memoize :is_ip_banned?, :is_proxy?, :is_recent_signup?
 end