FIX: 500 error when creating a user with an integer username (#16370)

Via the API it is possible to create a user with an integer username. So
123 instead of "123". This causes the following 500 error:

```
NoMethodError (undefined method `unicode_normalize' for 1:Integer)
app/models/user.rb:276:in `normalize_username'
```

See: https://meta.discourse.org/t/222281

app/models/user.rb

@@ -273,7 +273,7 @@ class User < ActiveRecord::Base
   end
 
   def self.normalize_username(username)
-    username.unicode_normalize.downcase if username.present?
+    username.to_s.unicode_normalize.downcase if username.present?
   end
 
   def self.username_available?(username, email = nil, allow_reserved_username: false)