users: add user name blacklist.

app/logical/user_name_validator.rb

@@ -6,5 +6,6 @@ class UserNameValidator < ActiveModel::EachValidator
     rec.errors[attr] << "must be 2 to 100 characters long" if !name.length.between?(2, 100)
     rec.errors[attr] << "cannot have whitespace or colons" if name =~ /[[:space:]]|:/
     rec.errors[attr] << "cannot begin or end with an underscore" if name =~ /\A_|_\z/
+    rec.errors[attr] << "is not allowed" if name =~ Regexp.union(Danbooru.config.user_name_blacklist)
   end
 end

config/danbooru_default_config.rb

@@ -69,6 +69,11 @@ module Danbooru
       true
     end
 
+    # An array of regexes containing disallowed usernames.
+    def user_name_blacklist
+      []
+    end
+
     # Thumbnail size
     def small_image_width
       150

test/unit/user_test.rb

@@ -91,6 +91,13 @@ class UserTest < ActiveSupport::TestCase
         assert_equal(["Name cannot begin or end with an underscore"], user.errors.full_messages)
       end
 
+      should "not allow blacklisted names" do
+        Danbooru.config.stubs(:user_name_blacklist).returns(["voldemort"])
+        user = build(:user, name: "voldemort42")
+        user.save
+        assert_equal(["Name is not allowed"], user.errors.full_messages)
+      end
+
       should "be updated" do
         @user = FactoryBot.create(:user)
         @user.update_attribute(:name, "danzig")