Optimize uri normalization (#4212)

* Add dependency on idn-ruby to speed up URI normalization

* Use normalized_host instead of normalize.host when applicable

When we are only interested in the normalized host, calling normalized_host
avoids normalizing the other components of the URI as well as creating a
new object

app/lib/tag_manager.rb

@@ -70,7 +70,7 @@ class TagManager
 
     uri = Addressable::URI.new
     uri.host = domain.gsub(/[\/]/, '')
-    uri.normalize.host
+    uri.normalized_host
   end
 
   def same_acct?(canonical, needle)