FEATURE: Anonymize User. A way to remove a user but keep their topics and posts.

app/controllers/admin/users_controller.rb

@@ -22,7 +22,8 @@ class Admin::UsersController < Admin::AdminController
                                     :remove_group,
                                     :primary_group,
                                     :generate_api_key,
-                                    :revoke_api_key]
+                                    :revoke_api_key,
+                                    :anonymize]
 
   def index
     users = ::AdminUserIndexQuery.new(params).find_users
@@ -333,6 +334,15 @@ class Admin::UsersController < Admin::AdminController
 
   end
 
+  def anonymize
+    guardian.ensure_can_anonymize_user!(@user)
+    if user = UserAnonymizer.new(@user, current_user).make_anonymous
+      render json: success_json.merge(username: user.username)
+    else
+      render json: failed_json.merge(user: AdminDetailedUserSerializer.new(user, root: false).as_json)
+    end
+  end
+
   private
 
     def fetch_user

app/controllers/users_controller.rb

@@ -93,7 +93,7 @@ class UsersController < ApplicationController
     guardian.ensure_can_edit_username!(user)
 
     # TODO proper error surfacing (result is a Model#save call)
-    result = user.change_username(params[:new_username], current_user)
+    result = UsernameChanger.change(user, params[:new_username], current_user)
     raise Discourse::InvalidParameters.new(:new_username) unless result
 
     render json: {